Yes, be careful of those leaves. The back injury that kept me off the bike for over a decade may have been caused by a crash that was the result of leaves on a trail. I say "may" because it was several years after before I was in "bad" shape and that was the only acute injury I could recall.
It was a fairly fast down hill with a deep water puddle at the bottom...leaves floating on the top of the puddle kept the puddle hidden. And when I hit the bottom at 35, or so, mph I took a nice flight over the bars (with a not-so-nice landing).
My back was sore for a couple weeks...didn't think much of it, though I did take the rest of the season off. When I started riding the next spring, I started feeling tightness in my back that got worse 'n worse over the next few seasons (!!)...until one day I couldn't walk. The docs thought it was muscle spasms. They finally did an MRI when I couldn't walk and my toes were numb. Two bulging, herniated, and compressed discs. Joy.
But hey, I'm riding again! Very leary of those leaves, though