The Principle of Least Astonishment suggests that a system should operate as a user would expect it to, as much as possible. In other words, it should never "astonish" the user with unexpected behavior.

In your experience as the "astonishee," what types of systems are the worst offenders, and if you were the project manager, how would you correct the problem?

Bonus if your answer describes how you'd retrain the developers!