Helpdesk callers who say "It just went funny all of a sudden" and expect you to know what's wrong, presumably by figuring out telepathically what they did to it that they're not telling you.

How about the ones that claim to have done something "over and over" - but when you walk them through it it works? Or the ones who say their usename was ALWAYS xxxyyyzzz when the creation date on their account as aaabbbccc is five years ago?