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Depending on who you ask, Bumblebee's mouth looks like a gag or a pacifier.
A common joke is that Optimus' entire character arc is him slowly going from somewhat of a Martial Pacifist to a brutal war criminal who casually mauls his enemies, usually in the head/face area. Some fans try to justify it as his 5 or so years on Earth making him lose his morals after spending hundreds of years as a noble warrior. But even ignoring the nonsensical logic of that, it's still hilarious to watch him rip a Decepticon's brain out while shouting "YOU DIE!!"
Sam's hilariously over-the-top, melodramatic reaction to finding out that Mikaela has a criminal record. Not only is this not something that's a huge deal in and of itself, but all of the Autobot-military stuff going on around him right then makes it look unimportant by comparison.
The Autobots and Decepticons discovering Sam's trying to sell a MacGuffin on eBay is supposed to be funny. The complete seriousness the robot cast has over it and this being the catalyst of a massively destructive war takes it from funny moment to unintentionally being hysterical movie-wide.
Bonecrusher's Angrish comes off as awesome and terrifying, except when he transforms, for which you will find many people who think that he says "WANT TEA".
During the otherwise spectacularly rendered forest battle, there's a very glaring animation shortcut, where, for a split second, Megatron goes from charging furiously at Optimus to strolling complacently. It's partially obscured by the nature of the shot, but many sharp-eyed viewers found it hilarious.
After the forest battle, Megatron and Starscream meet atop a building to discuss their plans. While the orbiting shots of Megs stomping his huge tank feet and growling make him look menacing, the sight of him clenching the fists of his comparatively tiny arms makes him look more like a T-Rexnote No, not like that!.
Megatron dramatically yells "All. Hail. Megatron!"... At a bunch of elephants and zebras. The implication there seemed to be that Megs was a tad delusional by that point, given his horrific head injury and all. Still, at least it wasn't as bad as it was in the novel and comic, wherein he follows up said yell by randomly blowing up an elephant with his fusion cannon.
The "RUNFAAAGH!" used by Sentinel towards the end of the final battle. The fact that's it's a stock sound effect from the previous film, from Megatron no less, doesn't help.
Sam's complete meltdown and utterly ridiculous screaming when the security tries to impound his car.
The scene in which the Autobots return is marred slightly by several factors:
Optimus announces his return by matter-of-factly declaring that "We will kill them all," before siccing the Wreckers on a Decepticon drone, who spout flippant one-liners while they tear said bots limb from limb. Given Prime's attitude in the previous (and following) films, it comes off as less "O.O.C. Is Serious Business", more "here I go killing again."
Optimus then proceeds to launch into an inspiring speech where, due to some Ambiguous Syntax, Optimus implies that he let thousands of humans die in order to prove his point. note Optimus declares that "Your leaders will now understand: Decepticons will never leave your planet alone. And we needed them to believe we had gone. For today, in the name of freedom, we take the battle to them." Naturally, he's referring to the Decepticons in the speech, but thanks to the ambiguous wording, he makes it sound like the humans needed to believe they'd left.
And while inspiring music plays, Bumblebee transforms and stops in front of Sam, inspecting him. They awkwardly stare at each other for several seconds before Sam moves the scene along by asking how the Autobots survived the shuttle explosion.
Starscream's utterly pathetic death. Even disregarding the silliness of Sam of all people being able to take out the second-in-command of the Decepticons, Starscream's behavior during the scene (flailing about wildly, spitting out oil(?) whenever he opens his mouth) makes it impossible to take seriously. Then again, given that it's Starscream, this may be the point.
When Cade asks Savoy if he has a warrant, Savoy replies with "My face is my warrant!".
Optimus literally growing Kibbles and Bits when scanning his alternate mode. There's a reason why the camera usually cuts away when characters scan their alt modes...
Optimus flying to space. It comes out of nowhere and is an immediate WTF. You'd almost expect the Superman: The Movie theme to play.
The finger-like protrusions that form Lockdown's collar are shown wiggling a few times. It's very silly-looking and clashes with his general visual style.
In a similar fashion, Hound's clothing can be rather hard to take seriously, particularly the upscaled dog tags and pouch vest.
"You see my face, your life is done!" Especially hard to swallow as this particular bit of nonsense loses Lockdown the fight at the end.
Worse still in that when someone's life does end at Lockdown's hand, it's because he turned his face into a sniper rifle and took a shot from a hidden location
The humans teaming up with Optimus and Bumblebee to take down Lockdown? Kinda cool. Doing it all while Imagine Dragons' tie-in song blares in the background, complete with vocals on full blast? Cringeworthy at best.
When the Decepticons of this film have strange names like "Dreadbot", there's something hilarious about Megatron sounding dead serious when reciting their names.
The entire Suicide Squad (2016)-esque introduction of the imprisoned Decepticons is supposed to show them as more dangerous than the regular Decepticons we've seen before, but instead it comes off as derivative and corny. It doesn't help that one of them was brought in for bank robbery.
The Autobots' reunion is dramatic... and then you see Optimus do something as mundane as jogging, which comes off as hilarious.
Megatron blasting fire from his fusion cannon when Barricade breaks some not so good news to him. It'd look badass if:
It didn't happen three times within the span of 30 seconds.
It did anything other than function as a glorified flamethrower.
Megatron did more than lazily scorch some scrap metal while lounging on his throne made of wrecked cars.
Sir Edmund Burton making a Heroic Sacrifice would probably be suitably dramatic, if Anthony Hopkins wasn't recorded looking lost and confused as if he was completely oblivious to the battle going on around him or Megatron pointing his BFG at him. And then he's shot in a way with PG Explosives that just flings him off of his feet in a side flip with dramatic slow-motion before flopping over from his fatal wounds.
