i hate the one that appears when you've written a really long opinion type answer and click submit to find... NOTHING. it's ok if it's a copy n paste thing, but if you've done a smashing job on an opinion, then the second attempt isn't even half as good.
i find it's cousin, the duplication gremlin, rather funny. you click submit and two, three, four... or more are there! answers, comments, opinions...