I remember one time that I was in a traffic jam in the city that I was not familiar in. There where two lanes. I needed to go to the right so I went to the right lane. I waited and waited. The left lane already were driving. And then the car in front of me turned off his lights. The person left the car. Weird… I was between parked cars. I was there for a good 10 minutes.
For jacks I always assume that it can break at any moment. That is why I put the spare tire under the car when I have the car lifted. If the jack breaks, the car will fall on the tire and not on my face.