"several clients send me letters adjusting the form 2210..."

Seems if this were a inherent PS programming problem, it would affect more than just several, wouldn't it?  So the answer lies in the commonality of these several clients.  What were the states or locations where they lived? 

Every one of my clients who received a fourth quarter 2210 penalty adjustment lived in a Federally declared disaster area...course most of my clients live in MA!