Much has been written about how harsh these are, but I always took it as just one of those generally-held beliefs without ascribing to it myself.
Anyway, I can't think of much more harsh than this Japanese tongue-twister competition.
http://www.glumbert.com/media/tonguetwister
The guy in green is definitely my favourite, although the red guy is totally bonkers so deserves applause too.
[Edit: Previous title wasn't getting any attention]