This is what happens. A broken derailleur hanger. I've been in the bike industry long enough to know that it seems as if virtually every frame design has its own unique hanger. When this guy came into the shop and asked if I could fix his bike because his derailleur mount was broken, I told him that there was probably a 99% chance I couldn't after asking what kind of bike he was riding. But I told him to bring it it and I'd see what I could do. At the very least, I could turn it into a single speed.
He was riding to New Jersey from San Francisco via Seattle. Seemed a bit late in the season to be undertaking such a trip. But, heck, it wasn't me. He brought his bike in and I could see that the whole derailleur had spun and also broken the derailleur bolt pivot stops as well. His Specialized Transition also didn't seem to be the ideal touring bike, but again, it wasn't me.
So I set about checking some of the various hangers I've collected over the years and found one that might work with a little bit of (okay, a lot) filing. I got it to fit both the hanger bolt and the wheel axle. So what if it hung off the back side a bit.
Had to grab hold of the B-tension plate and spin it so it was properly tensioned and tighten the pivot bolt at the same time. I got lucky and got the threads started on only the second attempt. Got the derailleur installed, wheel back in place, adjusted the limit screw (requiring almost 3 full turns!), and removed the two halves of the spoke protector.
And as evidence that looks can be deceiving (becuase his bike was just one huge grimey, filthy mess with STI shifters that looked like they had been crashed multiple times, massive gaps in the bar tape, no bar end plugs...), I ran it through the gears and with a little lube on the chain, it worked perfectly. I told him he should get a proper hanger the next time he's in a town that has a Specialized dealer. He's got a way to go since he wants to stay on Highway 1 with no deviations for something so trivial as making sure his bike is up to the task of transporting his butt cross country.