Fudgability is next to Usability
A good post from the blog Kasei tells the story of a system so over-engineered that users couldn't use it. It's a good lesson in the KISS principle.
The Importance of Fudgability
"In any human process there's always a degree to which the outcome can be fudged by the person performing the task. Even when the rules are simple or well-understood, there are always cases when someone will have a compelling reason to do things differently. In this case we didn't even know all the rules, and discovered to our horror that there were many more edge-cases than we'd imagined."