I've had this happen on another forum, it was always one letter or punctuation mark that caused the problem. What I ended up doing was trying the post a line or sentence at a time, and when I got to the 'forbidden' part breaking that sentence down until I found the culprit - and then generally there was a way around it using spacing or a different word. (I think the biggest problem was a "/" pointed either way - sometimes that's seen as coding. Also I remember once I couldn't use "cc" as a shorthand for credit card. Weirdness, really.)