Why do you think they call it "Fall"?
Those wet leaves of autumn can be treacherous, on or off the pavement.
Today, I fell going downhill on a singletrack trail. My front wheel slid out when it transitioned from soft dirt to slick wet leaves. I rather gracefully leapt to my feet with the motion remaining after I slid a few feet. No damage, except I was wet and dirty for the rest of the ride.
Last fall, my wet leaf crash was on a paved multi-user trail. My stepson was riding fast about two bike-lengths ahead of me. I yelled, "Leaves--look out!" just as he started to slide. There was no room to either side on the trail. I locked my brake, but I too had to slide out in order to avoid running over Jerry. My language would have shocked you! Jerry and I had fun comparing our road rash and bruises by the side of the trail.
Wet leaves ranks up there with goose poop when it comes to slipperyness.
I went down on cobblestones that were damp and had leaves on them as well. Man, cobblestones are very hard and hurt a lot. I rarely use profanity, but when you think you just broke your elbow and are 20 miles from other transportation, it can slip out. I did manage to only to use one word though.
I was behind a guy that went down on leaves yesterday as well. Good for him I cary first aid supplies. He had a sweet road rash on his arm.