I was wrenching at the shop yesterday, and someone came in with a mangled front derailleur.

Further diagnosis indicated a worn-out BB, specifically a pitted spindle. So the wonky BB had produced enough play in the crankset so that the chain or big ring had caught and twisted the front derailleur.

We replaced the FD, the BB spindle, and installed 22 new 1/4" balls. Total parts cost: $30; 60 minutes of labor.