Things that shouldn't break games, but do.

[SimuLord]

Fallout 3: operation anchorage can completely break the game if you have the chinese stealth suit and the silent running perk. you are effectively invisible & undetectable

Add in a Perforator (silenced assault rifle) from The Pitt and you have my character's main loadout.

 

[Mikeyfell]

the sniper rifle in Mass Effect 1
If you've fired it once the only reason to fire anything else is to challenge your self.
and if the combat did ever get a bit clusterfucky busting out the sniper will immediately put an end to all troubles, even on Insanity difficulty.

also being evil in Fable. or being good in Fable
or having your controller turned on while playing Fable.
maybe Fable's just a broken game.