No matter how clear your design is, people will make mistakes. Your UI should allow for and tolerate user error. Design ways for users to undo actions, and be forgiving with varied inputs (no one likes to start over because he/she put in the wrong birth date format). Also, if the user does cause an error, use your messaging as a teachable situation by showing what action was wrong, and ensure that she/he knows how to prevent the error from occurring again.