Ahh... Nellie alerted me to this the other day, but I've been horrifically busy on a client's site until today.
The problem is with the forum software's censoring algorithm disagreeing with the server's. I had the problem before with another forum for canine immune deficiency sufferers (and their owners) - posters often needed to state the gender of their dog, but using the words "dog" and "s.e.x." (yep, without the dots, that gives me a 503!) in the same post made it get all upset for some reason!
I'll see if the SMF people have come up with a cure yet.....