I cycled through some leaves, once. There was a big heap of them right in my path. As I went over them I hit something very solid (a brick, I think) buried beneath them. The jolt nearly knocked me off the bike. I checked for damage, but couldn't see any. Over the next few weeks, the bike got harder to ride, the gears skipped, the crankshaft wobbled. It turned out that the rear axle was broken, and the knock-on effects damaged the whole drive train. Result: bike written off as not worth repairing.
Yes, you can't be too careful with leaves!