I was putting a rear end in a 70 challenger ,there was no engine or tans in it at the time ,had front tires on it ,jack stands in rear ,it rolled forwards when i was wrestling the axle under it ,i was on a creeper ,i was pinned under the car ,thought i would be crushed at the time ,i was ok but i was holding the car up ,yelled at the kid to put the jack under the car ,he wanted to run for help ,i was like get back here and jack this thing up ,i will tell you where to place the jack ,got him to jack the car up ,i put the stands back under it ,but blocked the front tires this time and finished the job .i was on level shop concrete floor and it still rolled over .How i was not crushed i do not know ,was flat on my back pushing the car up with my arms .but could not roll out on my own as i was supporting the car .