Mine are pretty tame compared to the other posts, but here goes.
1. Probably the absolute stupidest thing I've done--I wasn't even riding at the time. I was a skinny 5'10" 145 pound 20-year-old when I bought my first big bike: a shiny, brand new '81 CB900F. Previously riding experience had generally been limited to bikes under 200cc. Got the bike home from the dealership and began to wonder how much gas they had put in the tank. No gas gauges on bikes in those days, but I had learned (on much smaller bikes) that you could take the gas cap off, look in the tank for fuel level, and if it was really low on fuel, you could rock the bike back and forth to look for splashes in the tank. So I took the cap off, and looked in the tank, but couldn't see any fuel. I started rocking the bike back and forth to see if I could see any fuel splashing around, but unfortunately, I was not accustomed to doing this on a 600 lb. bike. I rocked it a little to far to the left, but by the time I realized I had gone too far to bring it back up to center, it was too late, and all I could do was keep my left leg out of the way and let the bike down on its left side as slowly as possible. Broke the left front turn signal, and scratched up the left crankcase cover, and I had only owned my shiny new bike for about 10 minutes. Worst part was my friend, who had given me a ride to the dealership and followed me home, was standing there watching me make a fool of myself and laughing at me.
2. Couple years later, same bike, had been drinking all evening with friends and we were nearly out of beer, so I decided we should go on a short beer run. Grabbed one of my friends to ride on back to carry the beer home from the nearby store, and decided to get cute after we got out of the driveway and pulled a wheelie. Nearly went over backwards. That was the very last time I ever attempted a wheelie. Also one of the last times I ever mixed alcohol and riding.
3. Still same bike, roughly same time frame, regularly making the 40 mile commute to college with the speedometer pegged at 85, and calculating my speed (generally in excess of 100mph) off the tach. Never had any mishaps, but in retrospect, riding that fast that often was probably pretty stupid.
4. Taking up motocross for the very first time at age 40. Local motocross track had an Over 40 Class on their monthly race day that I raced in. Was never very good (didn't have to be in that class), but did manage to win several 2nd and 3rd place trophies. It was a lot of fun, but I gave it up after a couple of years because my muscles were so sore going to work the day after race day (and sometimes a couple of days after race day) that I decided I was getting too old for that young man's sport.
These days, I ride my 'bird like the old man that I am. Never any insane speeds, just the occasional acceleration burst to just barely into triple digits on backroads, then quickly back down to legal speeds.