One that has bitten my colleagues lately:

Auto-increment primary keys get reused when, say, you create a record, then erase it, then pull the power plug. Mysql does not save the highest number used, meaning that you turn on the machine again, add a new record, and it has the same ID as the previous one.

My very subjective impression is that every time I have anything to do with Mysql, sooner or later, some weird thing like that jumps out and bites someone in the ass.