A couple near-misses:
-I had the front of my FJ40 held up with a Hi-Lift jack. The little cotter pin that is supposed to hold the jack handle in its holder was broken and missing, so the handle could come out. As I lowered the FJ40, the handle started swinging violently like treeroot mentioned above and the handle flew out with enough force to knock me over or break my windshield. Luckily it did neither and instead hit my passenger side mirror and broke the glass. Lesson learned - hang on to that thing!
Come to think of it I still need to find a new cotter pin to put in there.
-Was installing 80 series front springs onto the rear of my buddy's 2nd gen 4runner for a little extra lift. Had to jack the rear end of the truck up really high to get the suspension to sag. Didn't like the looks of the setup and gave the truck a little test nudge. It tipped the jack over sideways while moving the back end of the runner over about 2-3 feet. Luckily no damage or harm done. Lesson learned - keep the lift as low as possible, and/or use extra stablizing bracing when needing to hold the position for a while!