Jack failure

I thought i'd mentioned it in a previous post here, but can't find it. When jacking up the car to do maintenance, i always put something under the car, so that if the car falls off the jack, it's easier to jack it back up, and, by the way, nothing under the car get crushed. I'm thinking of myself, for example.

Well, i was changing my oil, and while pulling hard on the oil filter wrench (usually i can just twist it off with my fingers), the car rolled back six inches, falling off of the jack (closest to the tire). Fortunately, i had the jack stand set up. And, it held it. In fact, the car didn't even seem to drop any. I say fortunately, but it's the way i work. Always have a backup.

The picture is kind of odd. The right front tire is shown, jacked up completely off the ground.

In case it's not clear, i was indeed, under the car at the time.