Back in highschool auto class we learned why it's important not to double gasket the oil filter. Kid had to clean up the mess not to mention the earful from the instructor.
Doing some maintenance on my first motorcycle I learned why it was important to have the front end properly support when doing any fork work. I wanted to change the oil in the forks. With the bike on the center stand I unscrewed one of the caps on top of the fork leg. Just as I got near the end, the cap and spring shot up at high speed. It hit the ceiling of the garage, leaving a nice gouge. Had I been standing over, it would of taken an eye out. Oh, and then the bike fell over
Cut myself almost to the point of needing stitches some years back while installing the catback on the G. Those heat shields are damn sharp! Patched myself up with a paper towel and some duct tape. Left a nice scar.