I noticed a lot of noise from my drivertrain and some weird pinging noises today whenever I hit a bump. I stopped and checked the bike once, didn't notice anything strange. Got annoyed and stopped again later to find that my cassette was loose as hell. Didn't have a lockring tool with me (who carries one of those on a ride) but got it tightened pretty well with my fingers. Sure seemed like a rare and strange problem to have out on the trail.