Presumed begotten Pangolins

Alastor Moody and Remus Lupin weren't incompetent though!
Ok the first one got tricked and replaced but the second one was just a gentle wolf.

you could argue that, while not incompetent as a teacher, he failed one test of competency by forgetting his wolfsbane potion, by the book explanation seemingly forgetting to take it as it was in-front of him