I just remembered one I did a few years ago on MSN Messenger, as it was called back in the day. I was talking to a girl I fancied and she complimented me. I then copied said compliment to someone else, in an 'Omg she just said this' sort of way, only to realise that I'd sent it to the girl in question.
I just thought of this now and the mental pain was incredible. I know it could've been far worse, though.