I'm riding a 200 km randonnee tomorrow, so yesterday I commuted on the bike I'm riding tomorrow; just to make sure everything is working. A mile before I got home, the chain skipped one time... no big deal, I didn't think anything of it, though it did seem strange, the drive train being pretty new.

So this morning I got on the same bike and started out... got a quarter mile before I realized it wouldn't shift at all. Stopped to take a look at the derailleur, and saw that the chain was routed through the derailleur cage completely wrong. It's supposed to go in front of the upper pulley wheel, and behind the lower one; but it was behind both. Very bizarre; I couldn't think of any way this could happen. I managed to ride home, not shifting, to take a look.

Well, it turned out the bolt through the upper pulley wheel had come loose, letting the chain slip past it before it snapped back into place. Pretty easy fix, but one of those little dish-shaped bushings had gone AWOL and I had to replace the whole jockey wheel. Five minute job; but I'd already missed my train and didn't have time to catch the next one either. So this cost me about an hour total.

On the bright side, I was able to fix this at home, where I had a spare jockey wheel; had this happened out on the road tomorrow, I'd be pretty out of luck.