A long-planned multiple-day walk I completed last September happened to coincide with a 24-hour triathlon that was taking place on part of the same route. I didn't actually meet any of the competitors but I followed some of the way they had run earlier, where the already very wet path had been turned into an absolute quagmire, which slowed me down considerably.
Not only that, but I innocently followed their obvious muddy trail where they deviated from the proper path and thus I followed a steep way that contoured a severe wooded slope, beyond which I had to pick my way across a rock field, all of which I should have avoided..