Meet the Common Merganser and it's toothed beak.
Granted, these are not real teeth but serrations on the rhamphotheca. But by all means, they work as well as teeth in helping the merganser to catch slippery prey, and surely could cause a lot of danger when biting your hand.
They also serve to give us a nice glimpse on how most enantiornithes and even deinonychosaurs could have looked like, all birdy and feathery but with nasty teeth on their mouths. Oh, nature!
Image source:
www.vancouverislandbirds.com/J…