With so many ways this can go south it's worth it, to me, to just get the points on my American Express!
http://community.cartalk.com/discussion/512009/scissor-jack-warning-mine-collapsed-this-weekend
yeah it does happen
"I was using my scissor jack to jack up my Taurus this weekend when it suddenly gave out - sending the car (with loosened lug nuts) swiftly to the ground!! Now, I wasn't going to get underneath the car until the jack stand was in place (it was waiting by the passenger door and actually scratched/dented the car slightly as it fell to the ground), but still - the sight of this collapse shook me up a bit. Looking at the scissor jack, there is this puny 1" long solid block-like piece that sits within the frame that the threaded screw passes through. It appears that the inner threads of this block just wore out, causing the threaded screw to slide right through it! Should've known something was up because the jack had gotten hard to turn at very low height (with no load). I had lubricated it a couple times with no real effect, but didn't think to investigate it any further."
also i remember watching on the news a while ago someone was doing oil change when the jack failed
"I use multiple methods since I'm super paranoid. I leave the jack under whatever end I'm working on, plus use stands (high quality locking), plus shove tires under the sides just in case the first two fail (jack loses pressure, stands slip out, etc). Sometimes I mix ramps in there as well.
I will never again use a cheap jack. Right now I'm using a $350 craftsman. I've had $100 and less jacks fail (which is what makes me so paranoid).
If I'm going to die in a car related accident I'd rather be piloting it than crushed by it."