THe skill system in Gamma World 4th. Followed closely by the item identification flowchart.
It's almost like a bait and switch, you start reading and everything seems nice and straight forward, almost like a proto-D20, and then suddenly, in the course of like one chapter, you're hit with a skill system every bit as counter intuitive as THAC0 was, and a hideously complicated flowchart that I'm apparently expected to go through just to figure out what a toaster is.
It really killed my buzz for the game. If I do run it in the future, houseruling the skills system to D20 rollover, and dropping that damn identification chart, will be the first thing I do.