For me personally it's the hyper-extension stand (wouldn't really call it a machine because it's a bodyweight-only piece of equipment).
Basically whenever I use it I always seem to be situated in such a way that it crushes my balls on every rep - no matter how far forward or how far back, my balls get in the way and the pain is
excrutiating.
Basically what I'm saying is that my nutsack gets to failure before my lower back does.
![Lips sealed :-X](http://www.getbig.com/boards/Smileys/classic/lipsrsealed.gif)