Clients who send you an email and then call to say that they've sent you an email are the worst.

Next: Mention on Twitter about it with a hashtag: #CheckEmail

I'd argue that clients who call you and say "Hi, I am sending you an email right now. aaaaannnddd.... sent. Did you get it? Allright, I'll wait. How about now? Oh, great. So please reply to this mail thread ASAP, okay? thank you. Bye." are a tad worse :)

They want their email to be read first.