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If two objects override hash to return the same value (either on purpose to indicate equality or by accident) and observers are set on both objects, the KVO mechanism treats them as the same object.

This can result in, for example, an observer getting notifications for properties it didn't set observers for.

See the attached test case for an example.

This appears to be due to KVOProxyMap using the object hash as the lookup key.

Perhaps there needs to be some kind of non-overrideable id property available on CPObject which is guaranteed to be unique and should be used in these cases instead of hash?

I believe there is a similar issue in CPCoding where two objects with the same hash can confuse things. I haven't worked up a test case for that one (yet).

An additional issue is that calling [self class] within hash on an observed object causes a stack overflow due to infinite recursion. This is due to _CPKVOModelSubclass class calling [self hash] but if a non-overrideable id property was used instead this should go away.