I thought about a warning, but the problem is that a subclass using the concrete API as part of its *implementation* of the associated slot or method is actually perfectly correct usage.

I'm not sure this is enough to give up on the idea of OrderedDict being a dict subclass implemented in C, though. It seems to me that some defensive internal checks (to pick up state corruption due to this problem) would be a lesser evil than giving up on being a dict subclass.