Anyone here surprised? No? I didn't think so.
Reposting because I know someone else is about to: http://en.wikipedia.org/wiki/Scunthorpe_problem
Consider: sh1t, cu|\|7, a55, f00ker, and so on and so on.
Plus you have to constantly update to take into account new slang which can have double meaning. Some kid might say "she's just a thirsty ratchet" and there's no way in hell that a simplistic automated system is going to be able to flag that as being a use of expletives or filthy and derogatory language.
I'm sorry, I know it's impolite, but I chuckled and nearly spilled coffee on my keyboard.