My worst one was around 6-7 years ago. On Sundays I used to do a fast-paced 25 mile group ride from a park in MD down into Washington DC and back. This one Sunday, a few of us were out in front, pacelining, but there was a new guy who didn't understand what a paceline was all about.
He'd sit in line and think, "Wow, I can go faster than this", and he'd pull out to pass. Once in the wind, he'd lose steam after a few seconds, but instead of falling to the back, he'd try to jam his way back in right where he was. This led to chaos and some tire rubbing, and a few close calls.
I finally decided that eventually, one or more of us was going down as a result of this guy, and I didn't want to be included. So I pulled off at an optional hill which I really enjoyed doing anyway. It was a straight, fairly steep climb, but the best part was the road back down to the parkway. It was twisty and had a series of four, slightly downhill curves of various radii that I could take at speed.
Once I reached the top of the climb, I waited for a few cars to get ahead of me, because I knew I could take the turns a lot faster than they could, and it sucks to get bunched up behind a 20 mph car in the fun stuff. Finally I took off, cranking down the straight leading up to the first two turns.
I took the first curve - a 45 degree right-hander - at around 40, then braked for the 90 degree left-hander that immediately followed. As I leaned the bike over I thought I might have been going a wee bit too fast, and just as that thought crossed my mind, my front tire washed out instantly. I hit the pavement hard, and slid on my left side straight into the shoulder, which was a dirt burm because the curve was carved into the side of a hill.
I went head over heels when I hit the dirt, finally getting spit back out onto the road to slide some more on my back. Since it was a blind curve, my first thought was to get me (and my bike) off of the road. As I hobbled over to get my bike, I almost slipped and fell again. There was something on the road, and later (after smelling it on my arms) I concluded that it was a mix of oil and anti-freeze.
My reward was significant road rash on my left forearm, upper arm, and left hip. I also had some puncture wounds on my right side from debris that were on the shoulder of the road, and I was covered with dirt and filth from rolling around. The bike was a little dinged up (STI lever, mainly) but ridable and otherwise undamaged.
I had to ride back to the start of the ride, and when I got there, there was someone with a camera. I guess I looked pretty bad, because he refused to take a picture of me.