Let me tell you a story too:

I was working on a V8 Buick Wagon, replacing bits at the front of the block (water pump I think) with the car on jack stands, and firmly positioned too.

The guy I am working with needs to replace the rear brake shoes, so he gets a floor jack and lifts up the back end from one side. The parking action of the transmission was no longer effective...

The car gently rotated across the jack stands, fell off them and crushed the stands to about 2 inches high.

I had been under that car about 2 minutes earlier.

Never ever lift a car when somebody is under it, even with secure jack stands.

The stands were made from two tubes, the outer one being split into three, splayed out and had a bit of welded metal to form a ring round the three legs about 1 inch from ground level. To look at they were solid and well within their load capacity (2.0 ton each).

Since that date I have never used that kind of jack stand again, I always use the ones with a wider base and a ratchet mechanism (WalMart sell 'em) and with a significant level of spare capcity. Also I leave the floor jack in place too and generally put the tires under the car. It all adds up.

You only get one chance...

PS the car suffered no damage apart from a slight dent in the heat shields round the front rotors; we straightened them with a screwdriver. Getting a jack under the front of the car was more fun though...