I learned a valuable lesson in high school one evening while working on the Belvedere, I had it up on ramps and was changing out a shifter cable. Our driveway had a very slight incline but it was enough to help the car roll off the ramps when I accidentally pulled the trans lever out of park while removing the cable. Luckily I got out from under the car in time, but my sisters car which had just been picked up from the body shop 2 hours prior wasn't so lucky,
lol. It went back the next day to have that 24 hour old fender replaced again and I figured out why wheel chocks are so important. As I've gotton older I have wisened up and always use jack stands and keep the jack in place whenever possible as a little extra insurance.