OUCH! That would put me in a rage for sure. I have never had that happen but have always worried about it. I had to pull 2 3208 CAT diesel truck engines with mine a few days ago. They have to weigh close to 1000 lbs or better fully dressed. I double chain them for safety. I built it myself with a 110 wire welder years ago. I wanted to stress test it before ever trusting it so I lifted up the front end of my buddy's '73 Olds 98! It did not have an engine in it, but i'm sure that was still close to 2000 lbs. hanging there. My cheapo "cart casters" were shot so I put on 4 new steel wheel bearing-mounted greaseable ones before pulling those CAT motors. It now rolls easier with one of those on it than it did with NOTHING on it before!
I think the best idea I implemented when building it was to set the casters "up". The frame is about 3/4" of an inch off the floor. That way, if one of the casters buckles over or snaps off it won't topple.
As long as your engine did not land on the balancer or hit it hard you probably don't have anything to worry about. Same would go for landing on a man. trans flywheel. It does not take much to spring the crank out of straightness when this happens. You might not see it without pulling the crank & checking it in some V-blocks or a crank grinder with a dial indicator.
