Saw this on alt.peeves (a great source for inane griping), and I couldn’t have said it better myself.  So I won’t.

The phrase that peeves me is “My bad”; it’s just two adjectives stuck together, and in the contexts in which it is used, no such combination is acceptable. It’s just plain Bad Grammar.

via Origin of phrase “You have an excellent rest of the day” – alt.peeves | Google Groups.