I used a jackstand laid on its side as an expedient wheel chock while working on my Grand Cherokee; hey the shape was right so why not.
Forgot about it and backed the jeep right over it. I didn't notice it other than a mild bump- the weight of the jeep pancaked it flat. I discovered my error after I got back from my post-maintenance test drive! The bump I felt was so minor it never registered in my head when it happened.
Those jackstands are strong in one direction, but not sideways.
OP- you got a wake-up call. People have lost pets, neighbors, spouses, children, etc due to poor visibility below the hood-line or out the back. Thankful it was only a chair, antiquity notwithstanding.