^^^ Basically that's what happened to me. I'd recently replaced the old corrugated rubber slips with teflon strips and reversed (as I had always done), unhooked the boat, jumped back in & continued reversing. The boat must have started to slide, because all I heard was the missus yell out...so I slammed on the brakes!
...to much the amusement of the crowd of charter patrons and their skipper at the cleaning tables whom had just got back in from fishing. The boat was balancing nicely on the concrete ramp. Funnily enough, I pushed the boat (fibreglass) into the water by reversing the trailer into it gently - then the missus & I went fishing. The aft bottom point was crushed and although it let a bit of water in, the bilge managed it ok.
A few weeks later, glassed up, scratches sanded out & good as new.

I have seen a few sunken cars at ramps with boats on the trailers floating behind them - that's always worthy of some chuckles.