When word verifications resemble (or are) real words. A real example: "flurti". (BTW, why is the internet symbol for any kind of security the same as for wheelchair accessibility?! Something about that just isn't right...)
When I just keep refreshing the "post a comment" page on Vince Waller's blog and get a non-stop stream of randomly generated hilarious words. None have ever topped "pootomid", though.....