FDCPA.
A counterclaim for the debt is not a compulsory counterclaim in a FDCPA action based on a lender's previous illegal conduct in foreclosure actions.
http://www.ca6.uscourts.gov/opinions.pdf/15a0299p-06.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.