Not really on point here but if slot_num is something that already supports <=>, you can tidy up your method:
def <=>(other) self.slot_num <=> other.slot_num end
As for the original problem, are you working in the development environment? There are some odd issues that can creep into development that have primarily to do with which classes are cached and which are reloaded. The good news is that they don't (typically) effect production code. To feel safe, it'd probably be worth building some good unit tests.
