Quote Originally Posted by War_lord View Post
The point of the dice is to add a random element to the game in situations where leaving success or failure up to the GM's whim would cause problems.
And one of the GM's jobs at the table is deciding when that is.

Certainly for D&D, the ability of the GM to sometimes overrule the dice for the good of the game is explicit, including advice for how to do it (particularly in when and whether to roll secretly so players can't tell you're doing it) (Pg. 18 3.5ed DMG).