My OE, rust-free scissor jack in those exact circumstances folded on me. I even had lubricated the threads prior to using it. I thought I was imagining the car moving down while I was working on tire, but stepped back a second and "oh s#@+!" It was bending right in front of my eyes. Left lug nuts loose, called a friend to bring a pump jack.
Was on perfectly level surface, and prior to that incident the jack was nearly unused. My car was garaged over a decade before it was in my family, and the jack looked pristine. It's been turned into a lamp or something by recyclers now. I warn everyone not to trust theirs!