I had a learning experience last week.
I was using a Poulan Pro 20" chainsaw. As I was cutting the tops out of downed trees, I made 2 cuts, and was contemplating where to and how to set myself for the next cut. I was firmly on the ground and the saw was steady in my hands. I blipped it a few times and as I watched it, the bar, independantly of the handles, dropped an inch or 2" and into my leg.

Luckily, the bone kept it from cutting too deeply.
Further inspection revealed that a screw had come out of the vibration damper and allowed the 2 halves of the saw to move independantly.
It was bad enough I couldn't patch the leg myself and had to go get it stitched together.
The excavator driver said it was common with Poulans. I'm probably switching to Stihl. First I'd ever heard of this issue.
( maybe it shouldn't have been in striking distance of my leg, but it sure shouldn't have done what it did all by itself!)