All your base is belong to somebody else, and they set you up the bomb, so you have to launch zigs for maximum justice.
Any lock, whether on a treasure chest, door, or boobie trap, can be opened by any key. Unfortunately, each key can be used only once. The villians still somehow consider such locks to be an effective security system, despite universal keys being availible for sale in bulk in any town.
Nah, you could see it–it’d be hard to miss with all the wild gestures and head-shaking. You’d just never hear it. 
The final opponent at each stage will require approximately as much firepower to kill as was expended on D-Day. Why such an immensely powerful creature wasn’t sent agasint you earlier in the game when it could have squashed you like a bug need not be explained.
Oh, I can think of a few:
In any recent fighting game where everyone has a different ending, it’s pretty much a given that there’s only “one” real ending (unless it’s Tekken, in which case none of the endings have any reality to it). The easiest way to tell when an ending isn’t real is if doesn’t depend on that one character winning. For example, in Street Fighter 3 Third Strike, after Elena puts away Gill, she spend sthe next day in a cafe reminicing and responding to a letter. Question: Exactly how was her beating a fanatical self-proclaimed god a necessary step toward spending the next day in a cafe?
Any deaths that happen during an actual fight don’t “count” (this includes any and all death moves, like in Samurai Shodown). Only deaths that happen during intermission scenes, endings, etc. count (and even then, only sometimes). In fact, you can pretty much write off everything that happens during actual gameplay as “non-canon”.
Firearms in fighting games are about a thousanth as lethal as they are in real life. Guns, in particular, are little more than fancy noisemakers.
Shooting game accuracy: If the game is untimed, the enemies are dead-on and rarely, if ever, miss. If the game is timed, at least 90% of the enemies can’t hit the broad side of a freight train.
Also in shooting games, no matter how much time it takes you to get to the enemy headquarters/secret base/evil temple/inner sanctum/whatever, you always reach the final boss just seconds before he completes his diabolical plan.
I Thought That Was A Bit Too Easy Rule: In any stage except the first, if you manage to beat the boss really easily, it means that you’ve just beaten the first form, and you’re about to have a much, much harder second form to deal with.
If a racing game requires you to beat all the computer vehicles to keep playing, it’s an ironclad guarantee that exactly one of them will be an absolute rocket that runs on rails, and it’ll get stronger and stronger the more races you run without putting more money in.
The end boss will kick your ass and leave you in a near-dead state at least once in the course of the game, and may do so several times. He/she/it will always assume that you have thereby learned your lesson and won’t be causing them any more trouble.
The end boss in its final form will bear almost no resemblance to the end boss that has been kicking your ass throughout the game. It may grow wings, tentacles, or may even sprout entire naked women like buds.
There is always a happy ending, even if the entire universe has been depopulated by the end of the game.
You have the most powerful weapons in the universe, yet you die in one hit no matter what you get hit by. Until, of course, you happen to get the shields which CONVIENIENTLY happen to explode in around 5 seconds.
The hero’s hometown will always be burnt down.
You can sell you Kingly Great Sword of Utter Destruction anywhere, even a tarven which only sells ale.
Merchants have an infinite amount of gold pieces to buy your stuff and make a killing selling your weapons and items that cost insane amount of gold pieces but still complain that they are poor.
There’s always a wise old mentor who do something nice for you, such as identifying stuff.
The hero always turn up to the lost heir to a great So-And-So king.
A factory somewhere within the games keep producing the same items and armoury and weapons.
In a video game, a character never dies even though he/she may be roasted, freezed, stabbed, pierces, cut, torn apart, shrunk, petrified, shattered, diseased or posioned. But when the plot says a character must die, a single stab in the stomach will do the trick (FF7, anyone…)
You fought your ways through a dungeon of death-traps, evil monsters and other unsavory events but the villains always get there before you do, without triggering any of the traps.
Wild animals, like wild wolves, vorpal rabbits and tigers drop money and medicine.
Diablo 2’s Law of Unseen Container - a 500 metre tall creature can drop a 1000 metre tall great sword.
Diablo 2’s Law of Cloth Changing - Your can change from chainmail to full-plate in less than half a second.
Law of Weak Elementals - the elementals, such as fire, lightning and ice, are weaker in a video game because they are so weak, ie, lightning cannot electroute anyone outright.
The hero must fall in love with one of the party member of the opposite sex. Their love story will be a stormy one but before the ending movie they will be together again.
Gold is not a rare metal at all.
Video Game’s Law of Instant Digestion. The full benefits of eating vitamins and foods and drinks are reaped instantously after wolving them down one’s throat.
Unless one or both of them is tragically killed or turned to the evil side, as usually happens.
And speaking of being turned to evil, any character who goes bad and then redeems him/herself must die within the next thirty seconds, but not without giving some stirring speech first.
The mentor is supposed to be helpful, yet tells you stuff like “this is the Golden Pyramid” that you would know already.