A number of games include rules (or even house rules) for fumbling. Usually, on a roll of a 1, something bad happens. I'm not real fond of this. While it can produce some funny results, this rule is much more likely to hurt players than it is to help them. In general, players roll the dice a lot more than the DM. They are using the same character week after week. Having fumble rules affect them is much more likely to kill off their character than it is to help them in some way. I know at least a few DMs have an almost adversarial outlook while gaming, so they really like adding in random stuff like fumbles.
I don't really think this is necessary, especially if you are playing some kind of retroclone or any older edition of D&D. In these games, player mortality is already pretty high. Adding in such a random factor like fumbles is just icing on the cake. If a player dies, I'd prefer it be because he did something stupid or lost in a fight that he already knew was dangerous. Not because he dropped his sword and the blade sliced open his leg.
I could see using fumble rules for NPCs only. That would still add the comedic effect that fumbles produce, without actively harming my friends characters.