What's wrong with adding a little uncertainty to the game? Even the best at their job has a bad day occasionally. You've never seen a professional football player drop a pass that's right in their hands? Or a singer completely forget the words to their own song? It happens. Not often, but it happens.
As a DM, this frustrating situation comes up far too often:
Player: I want to tumble into a flanking position.
DM: Okay, give me a roll.
Player: I make it.
DM: But you didn't roll yet.
Player: I don't have to, I automatically make it.
At least with a "1" being a failure you make them roll the dice.
With that being said, I never let just one die roll be the ultimate determinant for a fumble. 5 percent is too much of a chance so, just like with a critical hit, I make the players roll again to "confirm". If they fail the check the second time as well, they fumble.
For a high level character, that means you really have to screw up to fumble.
