This time of year? Leaves.
You’re out enjoying the cool autumn weather, riding singletrack in a calm, quiet forest, when suddenly… BBBZZZZZZZZZZZZZZZZZZZZZZ. That frustrates me way more than it should, to the point where I’ll stop to fish out the leaves that are caught up in my frame, fork, chain ring, etc. And, of course, another 30-40 feet down the trail after taking off again… BBBBBZZZZZZZZZZZZZZZZZZ.