when I was 14 I was changing the wheel on my dads car and he didn't put the cheap jack on properly and the car collapsed on me fortunately I had turned to pick up the ratchet from behind me so I was fine, after that my dad bought a better jack. Then when I was 17 I was with a friend in his car and it got a flat whilst changing his wheel the car slipped a bit so I moved backwards and then the whole car collapsed, he hadn't put the hand break on. I am a bit anal about safety with jacks now.