They're constantly saving their best tricks until we've already done all the hard work for them, and even if not, then they behave inconsistently to the point of sheer idiocy. Here are the worst offenders.
Game: Halo 3
Character: Master Chief
In-game failing: Unreliable field equipment
MJOLNIR armour is rubbish. We're sorry, but it is. It might theoretically be a high impact, self-repairing, reflex augmenting shell of the purest battle safety, but what's the point of all of those features when it only works when it feels like it? Witness the example of Master Chief's two kilometre plummet from space as evidence of its inconsistency. It's no wonder he's the last Spartan alive if their gear is always on the blink.
And on a side note, he can fall that far without a crack and withstand the heat of atmospheric re-entry, but a few bullets will kill him dead? We hope he kept the reciept for that thing.
Game: Metal Gear Solid: The Twin Snakes
Character: Solid Snake
In-game failing: Highly inconsistent combat ability
Every so often, Snake displays a talent for hand-to-hand combat so brutal and balletic that it makes you wonder why he bothers with stealth at all. But if he could just remember to use his skills a bit more regularly... He would have been through Shadow Moses in about half an hour