I just discovered one of my bikes had a bent B screw. I was switching to a wheel with a 12-27 10sp cassette instead of a 12-25 and found the derailer cog was running at against the 27t cog on the cassette. That's when I discovered I couldn't add tension with the B screw because it wouldn't go in further. Took it out and had a look, and sure enough the screw was solidly bent. Took a screw from another bike and it went in fine, with perfect indexing and all, so it seems to be the screw only. There's no scratching anywhere to indicate any part has been hit. It's kind of a difficult place to kick or bump too without hitting something else, and the screw doesn't look particularly weak. I'm really wondering what might have caused this? I hope it's not indicative of a bigger problem!