I have had derailleur's fail but I was lucky because I carry the spare parts on me. What failed was the tension spring over time they loose the ability to keep tension on the chain and while you can pull them apart and place the spring in the second hole to give more tension that had already been done so the spring was at life's end when it gave way. The other parts that that I carry is a small plastic seal that keeps all the crud out of the tension spring housing and an o-ring that goes on the derailleur cage that the tension spring goes over to make a seal on the other end that the grub screw holds it all together. I also carry a spare derailleur hanger and alignment tool plus a set of jockey wheels but only because my tours end up being years long.