I don't come across this issue very often, but when a client brings it up, I know exactly what the problem is by the fact that it's so completely not obvious. Just yesterday, I had a client call up complaining that part of their massively long article was being cut off after form submission. No errors were thrown and the page processed normally, just with less data.
I encountered this for the first time maybe two years ago. It was such a pain in the butt to figure out. The problem was the Long Text Buffer Size in the ColdFusion administrator for the client Data Source. This is set as the number of characters that can be passed through ColdFusion in one textarea variable. That is the problem, but the REAL problem is that when you exceed this character limit, the server does not tell you that this is happening; it merely gives less data.