What An Idiot / Back to the Future

Traveling back in time sounds like a unique idea to meet the past and future of the exploits utilizing them. However, the idiotic moments caused Temporal Paradox to the character's choices.

The films

Back to the Future

• Doc and Marty have just reloaded the DeLorean time machine's plutonium chamber. Doc prepares to time travel 25 years into the future, but he sees the terrorists' van approaching.
  You'd Expect: Doc to jump into the time machine and yell for Marty and Einstein to quickly join him. The three could then hightail it outta there — especially since even with the time machine mods, a DeLorean should be able to outrun a panel van — and if worse came to worst, the three could quickly set the time circuits and go an hour into the future or past.
  Instead: Doc goes for his unworkable pistol, and this gives the terrorists enough time to drive up and kill him. Marty escapes to 1955 in the time machine, and ends up nearly erasing himself from existence.
• After Doc tears up the letter warning him about the Libyans killing him, Marty realizes that since he has a time machine, he can go back early and warn him.
  You'd Expect: Marty to travel back an hour early to give himself ample time to warn Doc.
  Instead: He only gives himself ten minutes, severely limiting his ability to warn Doc on time. Even if he can get to the mall on time, he's really cutting it close.
  The Result: Upon returning to 1985, the DeLorean stalls again, forcing Marty to run on foot to the mall, where he arrives just in time to see Doc get shot. Fortunately, Doc eventually read the letter and protected himself with a bullet-proof vest, but Marty couldn't have known that.

Back to the Future Part II

• Doc pulls up to the McFly residence in 1985 to pick up Marty to go back to 2015. He finds Jennifer there with Marty.
  You'd Expect: Doc to tell Jennifer that he needs to borrow Marty real quick for a science experiment, and that they'll be back in less than five minutes, which she would likely accept because she knows Doc. Doc could then drive Marty around the corner into an alley and fly the DeLorean away from there so she doesn't see them. There's no rush because they're in a time machine.
  Instead: Doc decides to let Jennifer tag along because he's inexplicably imposing San Dimas Time on himself and in a rush, and "couldn't just leave her there with that information" even though she would have no way of knowing that the car is a time machine. Jennifer ultimately becomes a liability in 2015.
  You'd Also Expect: Even with Jennifer in the car, Doc to pull around the corner into an alley and take off from there so no one on Marty's block sees them. There's no rush to depart because they are literally in a time machine.
  Instead: Doc decides to be cheeky ("Roads? Where we're going we don't need roads.") and takes off out in the open where anyone could potentially see them — and in this case, Biff sees them. As a result, Old Biff retains a vague memory of this in 2015, which leads him to spy on Doc and Marty and figure out that the car is a time machine.
• Shortly after arriving in 2015, Doc uses a sleep inducer to get Jennifer to go to sleep. They land in a back alley in preparation of Marty going undercover as Marty Jr.
  You'd Expect: Doc to take Jennifer somewhere — anywhere (or anywhen) — to keep her out of the back alley. She's pretty vulnerable in her state, and he's got a time machine, so he's got all the time in the world.
  Instead: Doc and Marty leave Jennifer next to some cubes of garbage in the alley. When Marty protests, Doc says it'll just be for a few minutes. It isn't.
  Then: After Marty escapes Griff when things go sideways, he starts making his way back across the street. He's immediately distracted by Terry, by the holographic billboard, and by talk of gambling. He decides to buy the almanac he saw.
  You'd Expect: At the VERY least, Marty to go check on Jennifer first.
  Instead: He goes straight to the antique store, and even after buying the book, he doesn't return to Jennifer right away, stopping first to marvel at his new purchase.
  Meanwhile: Doc returns to pick up Marty. He arrives by flying through the alley that they had landed in earlier, where Jennifer is.
  You'd Expect: Doc to get Jennifer first, since he's right there.
  Instead: He completely forgets about Jennifer, floats on through, lands on the street in front of the antique store, rubbernecks during the Griff situation, and only then remembers that they have to pick up Jennifer — which they don't do, because he gets distracted by the almanac. Because of the delay, Jennifer gets discovered by Officers Reese and Foley, who think she's the Jennifer of the future and decide to take her home.
• In 2015, when Marty and Doc go to the subdivision Hilldale to get back Jennifer, who was mistaken for her future self by a couple of cops, who brought her to her future self's home, Doc proceeds to go to the house on foot after parking the DeLorean, and tells Marty to stay and keep watch. Some time later, Marty spots an automated dog walker probe exclusive to 2015.
  You'd Expect: Marty to remember, "Oh yeah, I need to stay with the DeLorean! This is no time for distractions." and stay with the DeLorean, and not wander off.
  Instead (!!!): He lets himself be distracted by the automated dog walker in wonder and goes to check it out, as well take a look at the rest of the Hilldale subdivision.
  You'd Then Expect: Marty to grab the key from the ignition and close and lock the DeLorean's doors so that no one else can get in and use it, just like any other person with common sense would do.
  Instead: Not only does he leave it unattended with the key in the ignition, but he also doesn't close and lock the DeLorean's doors to prevent anyone from getting in and using it.
  The Result: Biff takes the DeLorean undetected, and travels back to 1955, where he gives his younger self a sports almanac, which results in him creating an alternate 1985 where he is super rich (due to him betting on the winning horses, thanks to info from the almanac), corrupted Hill Valley, had Doc committed to an insane asylum, murdered George McFly, and forced Lorraine into marrying him. And as a deleted scene shows, this comes back to bite Biff as well since it also results in Old Biff getting erased from existence in 2015, as it's likely that he was shot by Lorraine sometime in the mid 90's (possibly 1996).
• In 1955, Biff gives his younger self the sports almanac which creates the alternate 1985.
  You'd Think: Older Biff would make sure his plans worked out all right, and to help out younger Biff in case anything unforeseen happens. Or at the very least, he would keep the DeLorean for himself and not return it to 2015 right where Doc and Marty left it, because they're bound to go back to 1985 at some point and realize something is amiss. Or he could destroy it so that no one else can use it to undo what he's done. Since he has the time machine, he's basically holding all the cards at this point.
  Instead: He returns the time machine to 2015 at the spot where it was parked for no discernible reason, thus allowing Doc and Marty to go back and correct all the damage he's done, and all he got for it was being erased from existence (as seen in a deleted scene).
• During the third act, Marty learns that Old Biff gave the almanac to Young Biff in 1955 and that Young Biff earned his first million(s) in 1958 by betting on the ponies after turning 21. That gives Doc and Marty about a three-year span of time to steal the almanac back from Young Biff.
  You'd Expect: Marty and Doc to go back in time between the day Biff got the almanac and when he bet on the horse races when he turned 21 in 1958. That way, Doc and Marty don't risk running into their Part I selves.
  Alternatively: If they still think it would've been better to get rid of the almanac as soon as possible to minimize any changes to the timeline and because they probably don't know where else Biff is besides that date, to go to 1955, but wait until about 3am on the day of the Under the Sea Dance (after the storm had passed) and steal it while Biff is asleep or whatever.
  Instead: Neither option is considered and they go directly to November 1955, on the day Biff got the almanac, and Marty is told not to interfere with Old Biff giving Young Biff the almanac, to which Marty ends up having to steal DURING the school dance and risk running into his Part I self. All of the urgency of the third act in Part II is incredibly forced.
• After stopping Biff's buds from sabotaging the Johnny B. Goode performance by Part I Marty, Part II Marty needs to get out of there.
  You'd Expect: Marty to go directly to the school roof where Doc would arrive soon.
  Instead: He chooses to stick around and watch his Part I self talk to his parents. Biff then finds Marty and calls him "chicken." Marty then gets hit by the door by Part I Marty, and Biff takes back the almanac and hits Part II Marty twice (one for taking the almanac, and the second for ruining his car). Marty nearly gets himself killed getting the almanac back.
• After getting the almanac back, Doc and Marty go back to the starting area to burn the almanac and set things right. The lightning storm approaches, and Doc mentions that he'll need to take a special approach to land in the heavy winds.
  You'd Expect: Doc to attempt to make that approach and land the DeLorean after setting Marty down, especially since there's lightning everywhere.
  Instead: He doesn't bother to even try and land the DeLorean, and wouldn't you know it, the DeLorean (with Doc inside) gets struck by lightning when the storm approaches, sending him back all the way to 1885, where he eventually gets shot by Buford "Mad Dog" Tannen, Biff's ancestor.

Back to the Future Part III

• During the Evening Festival, Buford goes to murder Doc, but is required to hand over his weapons to Marshall Strickland and the deputy.
  You'd Expect: Them to also ask to check for any hidden weapons in Buford and his gang's hats. Even in 1885, you'd have to be Genre Blind to not check for any hidden weapons in clothing or hats (imagine that!).
  Instead: They don't bother checking if he hid anything in his hat, and let him pass. Unfortunately, he hid a small pistol and nearly succeeds in shooting Doc (which if it happened, he'd die from the wound in 2 days on Monday) if Marty hadn't thrown a Frisbie pie platter and caused Mad Dog to only hit Doc's hat.
• Doc realizes that he can't remain in 1885, so he goes to tell his Love Interest Clara that he will be leaving town, and she'll never see him again. Doc initially wants to keep the time travel a secret, but when she presses him for a reason why and demands to know the truth, he decides to tell her.
  You'd Expect: Doc to take her to the time machine and show her that he's from the future, or show her some future tech (like the hoverboard). She's smart and progressive for her time and would probably understand, since the future tech would be pretty hard to explain otherwise. Plus, if he takes her directly to the DeLorean, Marty would be there to vouch for Doc's word. Heck, Clara was originally supposed to die in the original timeline anyway (as the ravine was named after her in the original timeline after her wagon plummeted), so why not take her back to 1985 with them as Marty had suggested, as it would limit the potential damage to the timeline?
  Instead: Doc decides that he doesn't want to further alter the timeline, despite the fact that he had already done so by saving Clara's life. The lovesick Doc tells Clara the truth with no evidence other than his word, and she naturally assumes he's lying and tearfully tells him to get lost. Brokenhearted, he lollygags around the saloon and passes out at an inopportune time. The delay nearly costs him and Marty their lives, and nearly costs Clara hers when she finds out that Doc wasn't lying.

Spin-offs

The IDW comic

• Continuum Conundrum:
  • After using a steam-powered tricycle time machine to leave 1893 and get to the future, Doc accidentally overshoots 2015 by twenty years and discovers a future where everyone has cybernetic implants. The citizens of 2035 think he looks out of place in his 1890s clothes, and they don't take kindly to him lacking cybernetics.
    You'd Expect: Doc to brush up on future history and attempt a workaround, or use the trike to attempt a jump back to 1986 or to 2015. The trike's time circuits' calibrations are off, but Doc is smart enough to calculate the difference.
    Instead: He changes into his conspicuously outdated 2015 clothes, then attempts to access his safety deposit box at the bank. The bank teller gets suspicious when Doc doesn't have cybernetic implants, and a genetic scanner reads that his genetic age is twenty years too young. Doc gets arrested.
• Who is Marty McFly?
  • Marty is having an existential crisis due to his memories of the original (Twin Pines) timeline not completely meshing with events in the new (Eastwood Ravine) timeline caused by time travel. Professor Marcus Irving, an unsuccessful time travel researcher, suggests that Marty might fade away and be replaced by a Marty that lived in that timeline. Irving suggests that Marty write to Doc to seek answers.
    You'd Expect: Marty to realize that he and his Twin Pines memories already nearly faded away before, during the Enchantment Under the Sea dance, but ultimately didn't when George and Lorraine kissed. This alone means that his existence isn't a fluke; if he'd been meant to replaced by an alternate Marty, he would have faded away and been replaced right then and there.
    You'd Also Expect: He'd be a bit more leery of a complete stranger who has knowledge of time travel, especially because of what happened with Biff and 1985-A.
    Instead: Marty writes to Doc. Doc shows up. Irving steals the DeLorean's flux capacitor. A temporal arms race ensues.