Problem exporting a mixture of PAL and ProRes 422 in FCP
I've never encountered this problem, so unfortunately I can't give you advice on how to fix it. However, next time, I would suggest that you convert all your footage to the same format before you begin editing. Prores 422 is a great editing codec.

Go to this website and download MPEG Streamclip. It's a free program that converts your footage to different codecs. Much better than Compressor IMO.

I usually convert all my footage to Apple Prores 422 (HQ) and then, upon completion, export a Quicktime H.264.