Guide Dang It / Deus Ex

In Deus Ex, The Conspiracy isn't the only thing that can trip players up:

Deus Ex

• Certain ATM accounts will "restock" from level to level, meaning that the player can reuse the same login(s) at certain points (most notably, in the UNATCO breakroom, Hell's Kitchen Subway Station and the 'Ton Hotel) to siphon additional credits from the same account. Most guides and walkthroughs are not aware of this fact. Additionally, certain ATM combinations cannot be found legitimately, and can only be found by accessing the gamefiles (or finding a walkthrough where the writer has already datamined the relevant account[s]).
• The game presents a scenario where the player must either choose to shoot Juan Lebedev or let Anna Navarre kill him onboard his private jet at LaGuardia Terminal. However, there is a third, hidden path — shoot and kill Anna, at a point of the game where the player is led to believe the character has Story-Driven Invulnerability. Not only does this confer the most amount of skill points (200, versus 180 if the player shoots Lebedev, or none if Anna kills him), but it changes the tone of the following missions, as Lebedev reveals the extent of TheConspiracy and JC's true nature to him. Not helping matters is that at this point in the game, there is nothing to suggest the character isn't still invincible, as they are normally a tough opponent if they're made hostile.
• Midway through the game, when you finally meet Tracer Tong in Wan Chai Market, he tells JC Denton (the player) that one Dr. Feng (a Versalife employee who worked on the augmentations JC and Paul Denton have) went missing in the Canal Road Tunnel. The actual tunnel (that is, the one accessed from a passageway right beside Tong's compound) is not where you need to go. Nor is it the location the sampan trader refers you to in Tonnochi Road (the same place the German explorers are said to have been diving for treasure), a place that is actually connected to the place you need to go, but has pipes blocking your path through. Instead, the player must go to the Old China Hand bar (which is in another part of Tonnochi Road), access an unassuming vent in the bar's freezer area (which is difficult to get into because of problematic controls) and follow a long passage and ladder down to a flooded section filled with Karkians. However, doing so gets you another aug upgrade canister and rifle ammo, along with an Apocalyptic Log written by Feng.
• Several of the alternate and optional conversations in the game require the player to do things in Violation of Common Sense, such as waiting to talk to characters until the last possible moment in a level or backtracking long distances to hear different dialogue. JC has a completely different conversation with the captured scientist in the New West Coast Silo if you hold off on talking to him until after you've killed Howard Strong and are ready to exit the level (which means backtracking through pitch-black tunnels), while several of the citizens in Paris have different conversations about the police presence if you wait to talk with them until after you've met Nicolette Duclare, had the conversation with her at the helicopter outside La Porte L'Enfer, and refused to leave.
• Whether Paul Denton lives or dies is determined by a somewhat unintuitive trigger. It depends on whether you leave the Hilton through the front door or through the apartment window when a strike team is dispatched there. Whether you actually take care of the strike team is ultimately immaterial. You can kill them all but Paul still dies if you leave through the window, or you can rush past them and out the front door, which leaves Paul alive.

Deus Ex: Invisible War

• In Deus Ex: Invisible War, there is a way to complete both of the competing objectives given at Mako Ballistics, but this requires you to know that you can wedge a door open with a box (something that was also possible to do in the previous title, but never plot-relevant, nor did it involve a door that would permanently lock behind the player) and know that you actually can attack and kill the scientist who now has ten times the amount of health.
• Most of the special weapons in the game are in places the player is highly unlikely to miss (the Toxin Blade in the diplomat's apartment in Seattle, the Dragon's Tooth Sword in JC's vision, the Red Greasel Hunter at the bottom of the sewer entrance right by the exit from the Inclinator). The sole exception to this is the Hellfire Boltcaster, a variant of the regular Boltcaster which is hidden in a small room only accessible by jumping over to a small ledge in an optional area. The challenge is not so much seeing the room as it is getting over to it — due to the odd placement of the obstacles, players might not even be aware that they can jump over a ledge to get onto the awning (and the broken planks) that allow them to mantle up to the room to begin with.

Deus Ex: Human Revolution

• The introductory prologue and first mission proper is full of this, to such an extent that the only way to maximize XP gain is to metagame to an absurd degree.
  • The game doesn't tell you, nor confers any feedback, that shooting and killing the nine enemies during the prologue confers experience — but only if they've been shot in the head. Worse yet, this will invalidate a Pacifist Run from the get-go, which means sacrificing an achievement unless you're on a second playthrough.
  • The game completely averts Take Your Time (in stark contrast to the relaxed attitude at the Sarif offices) by making it so that if you dawdle around too long, the hostages at the manufacturing plant (in the first proper mission) will be dead by the time you get there. Nice Job Breaking It, Hero.
  • It's possible to eke out a large amount of experience via double-takedowns... but only if you're hunting for every Last Lousy Point of exploration/combat bonuses. Conversely, it's also the only way to unlock the "Social Enhancer" aug early (despite players believing for years it was impossible to unlock due to not getting enough XP for the second Praxis Kit), which is also the only way to get additional psychological information on Zeke Sanders, the final boss of the stage.
• The secret achievement "Lucky Guess" requires you to unlock a keypad by inputting an unattainable code. The panel in question is the bomb in the "Smash The State" sidequest and the code is "0000". A similar achievement in the following game, "A Heated Combination", which requires you to know that the unattainable code for the game's first keypad (also an Easter Egg / meta-example for players who've played prior entries in the series) is "0451".
• Rescuing both the captives and saving The Informant in the "Missing Link" DLC requires the player to know that (a) the options that the mysterious hacker (Garvin Quinn) tells you are not your only choices, and (b) that there is a room you can only get to if you either have the Jump Booster or carry a box into a vent with you to gain the necessary height.

Deus Ex: Mankind Divided

• A prominent example in Deus Ex: Mankind Divided is Marchenko's killswitch. You have two opportunities to get this item (which helps during the final boss battle if you're going the lethal route), but both of them are so well-hidden that you will not find them unless you have OCD-like tendencies of thoroughly examining every item and room you come across. The first is hidden in the G.A.R.M. facility — and in a first for this game (and likely the series), the only way to access it is to crouch-jump on top of a set of lockers, look down and press a hidden button wedged between the lockers and wall, which opens a compartment in one of the lockers that has a switch. The second is found in the final level itself — in a box hidden under a table. Up to this point, the game has never given an indication that there were items hidden in boxes, and the player is never led to believe otherwise. The second killswitch doesn't even show up on Smart Vision due to being inside another object, either.
• Getting the "best" outcome for the Dvali Enemy Civil War. Getting Otar and the rest of the Dvali operatives to remain civil when you visit the old Prague Theater in the third act not only requires you to have done all of the optional objectives that Otar gave you earlier in the game, but also knock out or kill Radich Nikoladze during the second visit to Prague, despite nothing in the story telling you to do so besides a couple of vague emails talking about Radich and Otar's relationship. Not only that, but completing the objectives in the opportune fashion requires you to use lethal methods (and thus break a non-lethal run). If you choose to let Louis Gallois escape (via faking his death and assuming a new identity) instead of killing him and his bodyguards outright, Otar and the rest of the soldiers will still be hostile when you arrive at the theater. This is despite him telling you earlier on that you've completed his favor if you do all the requisite tasks.
• The "Golden Rookery" achievement requires you to carry an oversized gold-plated pigeon from Ivan Berk's apartment right near the start location in Golem City to a hidden area within ARC Territory. Aside from the fact that Berk's apartment isn't even immediately noticeable from the ground (you have to climb several floors to get to it — the map doesn't help matters), the final destination requires that you've invested in the "Punch Through Walls" aug. And that's not even getting started on the bugs that can cause the penguin to disappear if you stray too far from it, especially within the Throat.
• In the "System Link" DLC, getting all the Praxis Kits (which are in short supply in the mission) requires the player to visit ShadowChild's secret room before you know she exists in the first place, at a point in the mission where Adam has never heard of her and no one else has mentioned her.