If I have got time to think, then anything that results in a breakfall/flip. Always been a fear for me, and something I have to confront every time I walk on the mat. Not sure why this is such a problem - there must be some history there somewhere but nothing specific I can recall.
The strange thing is I do them ok, and often get asked to uke for a visiting teacher.
I've been confronting this fear for 15 years and it's a persistent bugger. Anyway, I just keep training, and hope that repetition is the answer. I figure that one day it will just get bored with not winning and will quit pestering me all the time.
I have no problem as nage, though (any more than any other practice, that is) and see no correlation for me.