Derailleurs suffer catastrophic failures for really only three reasons, two of them very avoidable. In order of most common:
- Bent hanger. Riders often neglect to understand the importance of inspecting the alignment of a derailleur hanger. These things are easily bent, but easily inspected. When it does get bent, which could be as easy to do as bumping it on a doorway, the derailleur shifts into the wheel and causes all sorts of hell. Avoidable hell.
- The loose pulley screw. These little screws have been known to come loose, fall out, and then causing excess chain slack to clog the derailleur cage, wrapping it around the cassette. Again...totally avoidable with a little regular maintenance and inspection.
- The busted chain. Much harder to anticipate and avoid, sometimes chains just fail and again, cause the der to wrap around the cassette. Good chain maintenance and proper shifting technique (not shifting under load) can reduce the risk of chain failure.