One problem we've run into is that the browsers have internal limits on how large a form can be. nybooks has very long articles, and there are some which can only be edited with Netscape because it handles longer forms than IE does. There are some which cannot be edited via the web at all unless they are broken up into smaller chunks. This problem is somewhat insidious in that the browser will just truncate the form, and if you don't get any errors from missing variables then you'll happily go on, not realizing that the user's text has been chopped off at the knees.
If there are variables in the form after the long chunk of data we get the "problem with your input" message. That may be why you see this on forum posts, if you have very long-winded users. It would almost have to be a copy-n-paste situation, though; I think the limit is something like 32K, and it's not likely that someone's going to type in that much data in a forum posting.