I couldn't begin to say how many ringed shells I put through an old Iver Johnson Champion 20ga single shot when I young. We didn't have a clue what pressure was and probably wouldn't have cared. Fun to see a big yellow splat on a tree. Seemed like it kicked a bit more.
Fortunately nothing ever happened and I can afford real slugs these days.
I suppose it could be used as a field expedient if you were out bird hunting and got yourself treed by a big hog, bear or something.
In my experience it's been "No harm, No foul."