This thread on friendfeed is hilarious. At first I was reading through it and thought I must be missing some private joke. Further down, Lindsey supplied the quote from two comments that had been deleted, and it all came together.
The delete function is supposed to provide some measure of protection. Did you suddenly realize that your clever retort is not so clever? Did you make a comment and then notice that someone already said the exact same thing? Feel silly, all of a sudden, saying "me too"? Delete is there to rescue you. Or maybe not.
Someone, somewhere has seen your comment. Or, if it's one of Lindsey's threads, perhaps hundreds of people have seen the comment.
Note to self: Do not rely on delete anymore.