I agree, this is a dangerous thing to always watch for. It is especially wise to avoid any water or seemingly harmless debris, such as leaves in my case. It was the fall and I was feeling like a kid. (isn't that why we all ride???!!) In my town, we have drains that are just angles cement holes in the curbing. I wouldn't have done this on my narrow roadie tiree, but adventurous as I can be on my knobbies, I was plowing through piles of crunchy leaves on the side of the road when before I even knew it, my rear wheel was sucked sideways and down into the drain while the rest of me continued forward. I was fortunate to not have broken anything but my pride. Landing on one knee and an elbow, I also scraped my hip, tearing my tights and long sleeved izumi jersey. I still love to thrash the fall foliage, but believe me, I am much more cautious, and I NEVER ride right up next to the curb, when hitting those piles of leaves. I guess I'll never grow up. I just hope I'll be a little wiser!