When you type something for an instant message (ie. MSN Messenger) and after typing it you read it while hovering your hand over the Enter key. Upon reading it you decide to press Enter but right at the last moment you realize that:
a) you typed something wrong; or
b) your in the wrong conversation
But can't stop yourself from pressing enter in time, resulting in the message being sent anyways, much to your dismay.
Uh-huh, that looks good. Guess I'll send that. Wait a minute somethings wrong!