I got this idea while looking at the Movie Cast Ideas thread on the Current Talk '08 board. We know who you want to see, but who would you least like to see? I put this under the Games board because it's more "for fun" than the other thread. My only request is that, before you post your nightmare casting, you check to see if anyone has posted that person as their ideal casting on the Movie Cast Ideas thread; we don't want to step on feelings.
That said, a couple of my nightmare castings would be:
Barnabas - Steve Carrell
Julia - Fran Drescher