Hi,
I am very new to using the animation tools, so it's just a guess..... (Disclaimer here - lol) but i am wondering if you have 'mirrored' the model's arm and something has inverted too, something similar happened to me recently while attempting Mike's rigging tutorial, I had problems but did not get as far as moving all the joints, so I really don't know if it's the same problem, maybe not, maybe something similar?
I think I would try selecting various parts of the 'arm' and look to see if the axis of vertices/selection handles etc are pointing 180 degrees out from the rest of body or other arm.


I would have thought that it could be sorted out by selecting whatever, and rotating, so don't give up just yet, I'm sure someone here will know.

Good luck Scraggy