My first and worst self-inflicted cut was sharpening a machete (I worked as a surveyor for a while in highschool, good job) and had the tip pushed into the trunk latch of our work car. I had the butt of the handle pushed into my belly and the blade bend slightly from the pressure. I was sharpening it by pulling a steel file across the blade.
Yes, I know better now, but that was how all of the old guys did it...
A buddy of mine walked around the back of the car and accidently bumped my elbow at just the wrong time. My left index finger still has the scar where I pulled it up the freshly sharpened blade.
Then there was the time I was making thin lemon slices with a fillet knife. Fillet knives are flexible and should not be used for slicing, as I found out when a small correction in the line of the cut on my part caused the blade to bend over the tip of my thumb. Took the tip and part of the fingernail cleanly off and the lemon juice probably helped to disinfect it. Weird, but no scar from that one.
The worst I ever saw was a guy slip just a little while cutting bone-in pork chops on a band saw and basically split his index and middle fingers all the way up to his palm. The darned thing followed right between the bones and did not even cut tendons.
I'm sure you all have more interesting stories, but my point is that you learn from from your own mistakes and others as well. Here is the most important lesson I have learned so far: always know where your body is, where the blade is, and try your best to anticipate where they might meet.
Tip up or tip down, be aware.
Chris