Thanks for calling me in. I can confirm that the stack trace in https://paste.debian.net/361351/ points at a bug in Cython's own implementation (not the code it generates). It's deep-copying objects that don't support it (which doesn't normally have an impact because they aren't actually used). I'll try to fix that.

Given this specific case, i.e. deep-copying objects to death "by accident", however shows that this change is clearly a) helpful and b) harmful. It's going to help people write safer code, but it will also break existing working code that can't normally fail as it is and that people might not really want to (or even cannot) touch any more. Not really something for a backport, IMHO.