Actually, I think it is if you take to long to fill out the form???
You can definitely enter more than 140 chars and still get the confirmation screen. Other people have posted that they didn't get the confirmation screen, even with less than 140 chars (they actually counted). So they hit back on their browser (the form was still filled out), and then re-sumbitted and it worked.
Obviously, if you spent a couple hours filling out an essay, and then trying to squeeze it down to 140 chars, you probably ended up spending a lot more time on this then their default 'expiration' is set for... Doh!