I heard of a case where a FFL sold, not a PPT (consignments can be transferred as a PPT), firearm which was not on the certified list, the firearm had to be returned. I don't recall more, but somehow the DOJ found out. I am not sure that the customer could be forced to return it, but who knows.
FFL Transfer/Special Order Dealer since 1993.
Net-FFL list maintainer.
Never try to teach a pig to sing. You waste your time and you annoy the pig. - Robert A. Heinlein