[Boost-users] [serialization] unregistered_class exception that wasn't occurring before

Hello, I'm serializing the following type: (pseudo-definition) multi_index_container< variant< shared_ptr, shared_ptr, etc...

It works well when I make and load archives with the current version of my app, but when I try to open an archive made by the pervious version it throws unregistered_class for one of my classes. The both versions use boost 1.47, and the differences between them seem to me absolutely unrelated to serialization (as far as I can see). Unfortunately I can't provide here code snippets, so my question is theoretical: what could break the backward compatibility in such a way? What should I look into? Thanks.