In this case, a spring. Took the M1 Carbine to the range the other day, and today decided that it was probably time to clean out the bolt. And found that the extractor spring came out in two pieces.
I had a spare, so it's put back together; and now I need to pick up a new spare.
I will note that it the broken spring would not have been found without disassembly, as the extractor was working just fine. So no idea when it may have decided to attempt mitosis.