Several (28?) years ago, I was working on a car - a big Pontiac 2-door, replacing the upper control arm bushings. I had jacked the car up (floor jack) and put a cinderblock under the frame, on end, to support the car. I let the jack down, so the car was resting fully on the cinderblock, but I did leave the loose jack under the center of the front crossmember.
I then proceeded with replacing the upper control arm bushings. Part of the regime was to cut the rivet heads off the upper ball joint with a cold chisel. I had cut a couple off already. I was sitting, Indian style, with my head up under the wheel well, hammering away. All of a sudden, on one hit, the percussion of the hit caused the cinder block to shatter. The side of the car I was working on was much higher than the other side, and the car immediately fell and leveled out, rocking back and forth before it stopped. Had I not had left the floor jack under the car, I would have been decapitated, for sure.
After about messing my pants, I stopped the project and went out and bought proper jack stands. Stupid.
Ever since I have had a bad taste in my mouth for cinder blocks.