Mine broke twice, the first time it was the roll pin like the previous posts, the second time the knob broke where the roll pin sits in the grooves. Fixed it the second time by just getting a nut and screw for the tensioner system. Since the knob is useless now, I just need to use a ratchet and socket to adjust the seat - I don't think it's going to break again.
Oh, and by the way, I'm not heavy and didn't constantly adjust the seat - it just broke.