Hitman 2 and Hitman 3 features the briefcase, which can be used for the mundane task of smuggling equipment into secure locations, as well as acting as a melee weapon in a pinch to knock people out. It can also be thrown at peoples' heads for the same purpose. Problems arose when IO Interactive did not count on the briefcase being thrown at fast-moving targets (people on Jetski's, runners going round corners), and while they had adjusted the speed of the briefcase to not be so instant like other melee weapons, it was not even close to being fast enough to look somewhat realistic, so you could see the briefcase being thrown across a room, slowly curving to find its target, among other silly things. While IOI did patch this out four months later into Hitman 2's life, this bug proved to be so popular and memetic that it came back in the form of the "ICA Executive Briefcase MK.II", as a reward by completing the "Best Case Scenario" challenge pack, and the item restored the slow homing properties, and also slapped a worn-out "2" sticker on it, which itself was some Self-Deprecation at their MK.II line of weapons.
The NES version of the original Metal Gear had a glitch when you entered the computer room, if you went immediately right you would glitch into the final boss room without having to blow up the computer. This was helpful if you didn't rescue the professor (even though you knew you needed plastic explosives to destroy the computer, the game won't let you unless the professor told you).
Metal Gear Solid had the Instant Elevator bug. Rather than wait after calling an elevator, if you hit the button again it would arrive instantly.
It also has the Vent Glitch used by speedrunners. Switching to first person view while in a specific position in a specific vent causes Snake to go out of bounds. Guiding him then to a certain spot causes a cutscene to be triggered that advances the story to the part where Snake has been captured, allowing him to skip 5 bosses, at the cost of having a shorter lifebar.
When you quick-equip any item, it resets Snake's walk cycle. This means that by mashing the quick-equip button over and over you can sprint across noisy floors and puddles without making a noise since, as far as the game is concerned, you never actually stepped on that floor.
There is also this rather difficult to explain glitch that allows you to get up close and personal to Sniper Wolf in both battles. Normally if you try to get too close you are hit by a scripted, unavoidable attack that pushes you back several feet, but through a specific glitch involving a grenade and a ration you can render yourself immune to it, run right up, and punch her to death as she helplessly strafes around still trying to snipe you.
Though it's less of a glitch and more of an exploit, the developers not disabling the radar in the snowfield the second Sniper Wolf battle takes place in allows you to use Nikita missiles on her. As they only expected you to use the PSG1, she's not programmed to avoid, shoot down, hide from, or in any way respond to the missiles, allowing for an absolute Zero-Effort Boss. This glitch was so well-known that not only was it retained in all future releases and the Gamecube remake, but also inspired the trend of having deliberate Cheese Strategies for defeating all the sniper bosses in future gamesnote Metal Gear Solid 2: Sons of Liberty allows you to radio Snake for help during the brief sniper duel. Metal Gear Solid 3: Snake Eater allows you to use the Konami Code to reveal The End's position, wait a week for him to die of old age, or fatally shoot him after a cutscene earlier in the game (which, admittedly, is more difficult than the sniper duel, but still). Metal Gear Solid 4: Guns of the Patriots allows you to hide under the truck where you will never be seen or targeted, even if the opponent sees you go under it. Metal Gear Solid V: The Phantom Pain allows you to call a supply drop on Quiet's position and KO her instantly (or in two hits, on the highest difficulty).
Raiden has invincibility frames when he pops out from a corner and aims his pistol. If you get the timing down, you can outright No-Sell attacks from enemies by hopping out, firing a round, hopping back, and repeating. This is one of the most effective ways to battle Fortune and the Harrier, as you can just stand there like an arsehole dry-firing an empty pistol while they ineffectually spam you with their attacks.
There is one bug which, when combined with an Anti-Frustration Feature, allows you to skip the entireHold the Line battle where Raiden and Snake have to hold off waves of Arsenal Tengu. Before it begins go ahead and sucker-punch Snake a couple of times so he decks you back and you have less than half health, as you having low health will make Snake use his M4 Carbine instead of the USP during the battle. That's the legit part. Then, grab an Arsenal Tengu in a chokehold and drag him outside the room (through the door you came in) and choke him just once every single time he struggles, keeping him alive as long as possible. For some reason, as long as you're doing this, all the other Arsenal Tengu will leave you alone and target Snake, who can hold his own no problem with his M4. Then when the one guard finally goes down, take down the one next guard who comes in and the fight ends. You finish the entire battle by only taking out two enemies.
Cardboard is fireproof. Yes, by wearing the cardboard box you won't take damage from flames and can't be lit on fire, and even if on fire donning a box will put it out. The only thing that can hurt you is a direct blast from a flamethrower, and even that can't light you on fire. The glitch is because the developers never bothered to program an animation for Snake burning while in the box, presumably because they never assumed anyone would be dumb enough to use cardboard to shield from fire, and thus the game just ignores it.
Speaking of the cardboard box, Snake normally slows down considerably when running uphill or up stairs. With the box equipped, though, he will run at normal speed no matter how steep the slope is. This is very useful for getting good end-times for the game, as well as dodging and escaping guards who are still slowed down by these slopes.
When you pull the pin of a grenade there is a very brief window where Snake becomes both invisible and silent for all intents and purposes. With careful timing you can basically just run in front of guards without them noticing you and, since it's possible to put grenades away without throwing them, with practice you can just gallumph through entire areas without triggering alerts.
In Metal Gear Solid 4: Guns of the Patriots, a glitch with the long-barreled and scoped version of the Desert Eagle means the magazine he grabs to reload the weapon with is inside another long-barreled and scope Desert Eagle, which he duly inserts into the space occupied by the first. It helps if you exclaim "my deagle needs more deagle!" while doing this.
In Thief: The Dark Project there are underground giant bipedal lizards called burricks, which can be dangerous because they emit poisonous gas. While the game was still in development, though, bad coding led their attack to not have any range. That means, the gas was released on their place, damaging them, leading to what the developers commented as "farting to death". It was so humorous that they added it back in a special secret level for Thief Gold, dedicated to show to players the bugs and the scrapped ideas before game shipping with with comments from the team.
In the first two games, crouching and repeatedly tapping the movement keys instead of holding them down allows Garrett to move silently, albeit much slower.
, as a reward by completing the "Best Case Scenario" challenge pack, and the item restored the slow homing properties, and also slapped a worn-out "2" sticker on it, which itself was some Self-Deprecation at their MK.II line of weapons.