To the best of my understanding the correct contraction of "Am I not" is "aren't I". However, growing up in Scotland I very frequently heard an alternative contraction "amn't I". I think this though is extremely uncommon outside of Scotland.
Now obviously the latter uses the correct part of the verb for "I" and the former does not. I was wondering if anyone knew how we got the wrong part of the verb in the right contraction, and the right part of the verb in the wrong contraction?