The Light Action derailleurs have a square opening cut into the rear parallelogram arm, to accommodate the pivoted, spring tension arm. This is a weak point. I have seen several rear parallelogram arms break at this point. Failure normally causes the chain to derail and jam between the dropout and small cog. It's inconvenient, but usually not hazardous. There are probably many out there, still functioning well, but they do appear to have a higher than normal failure rate. If I had a spare, compatible derailleur to replace it, I probably would.