If you agree with my walking problem, then tell me that you have never mis-pronounced words from english vocabulary? It does not happen that often, but it does happen. So the spellcaster should miss the spell (@ 100%) sometimes on the basis of mis-pronounciation and loosing concentration.

Basic skills like Bash, trip & disarm can fail even when mastered as they are taught to almost most classes in one form or another. But specialized skills of an assassin (Kans, Kote, etc.) or a specialized warrior should not fail alot (once @ 100%) as they are very much specialized to that class/character only.

Example:
1. Very few people know how to operate a boat legally, they are called "Captain". Boating accidents do happen but its happens once in a while not everytime.
2. Not alot of people know how to use a gun. These people go to shooting ranges to master the art of shooting. Once they become very good, they do sometime miss but not 4-5 times if they shot 20 rounds.

The advicable fail rate should be 1% or less for the skills mastered (100%).