Although only a small part of one subject, my nemesis was Electrics. Ended up getting 70 first time in AGK because the CAA decided to throw in 3 times the usual number of Electrics questions.
Also, I must admit that despite passing subjects such as Radio Nav, Instruments and Met first time, this was primarily because I managed to remember the feedback and not because of some inherent understanding of the material. I think the current exam process is flawed.