Often users move by mistake base folders of their mailbox all around and so is not uncommon to end up with things moved where they should not be. Given time they forgot that "move" and so they call for help about "restoring" missing" elements.
I have had two users move their calendar in to their mailbox. This causes any client that views them in a multi-user calendar to crash before it is fully loaded. It takes quite a while to find the offender and drag the calendar out of their mailbox to fix it.