I've seen a few:
First was a bunch of guys with a grungy looking speed boat, they attempted what could only be described as a "life boat launch" on a public slipway, twat one reverses the boat back down the slip as fast as he can go, twat two's at in the drivers seat ready to gun the engine into life as soon as she hits the water, only twat one driving stabs the brakes far to soon and the speed boat shoots off the back of the trailer and hits the bottom of the concrete ramp, the noise was stunning, and alledgedly could be herd in the town, what i'd have given to have had a video camera that day.
Second was a guy attempting to launch his speed boat with a lenght of wo fully inadequate polly prop line, his wife guides the boat why the twit lowers it from the tow hitch on the car at the top of the ramp, only the ropes far to short so he decides to tie it off and reverse the car down a bit, so he's reversing away nicely as directed by his missus and she waves to stop and he stops but the nice lenght of polly prop rope snaps and away goes the boat with his wife still hanging onto the boat, boat and trailer launched perfectly except the trailers hanging off the bow attached by the bow line and the boats floating at a strange angle above, and funniest of all is his wife hanging on for dear life to the side of the boat. i nearly shit myself laughing at this one.
Another time, (and this is justice done) i watched a dick head jet skiier backing his ski on the trailer down the launch ramp, instead of reversing the car the dick thought he could walk the trailer down, the slip way was wet and greasy and off couse you guessed it he sliped and gravity took over, the trailer and ski took of and wiped out a second jet ski at the bottom of the ramp, before hitting the water turning turtle, you can image they were not happy bunnies.
Another slightly related incident i witnessed, donky Dad and his pikie kids with there 8k jet ski racing it off the bottom of the slip way, whole family watching as little jim bob decides he's going to pull a rooster tail down the inside of the inner harbour but instead of the thing going straight he's given it far to much power and the ski went verticle throwing him off the back and the ski onto the rock harbour wall
everyone in the car park ducked for the impending explosion
but the only explosion that day was from Dad as he dragged the knackered lumps of what was his prized jet ski back up the slip way.
Once watched a guy pull his boat and trailer out of the water and as he's towed it up the slip he has a lobster pot and 20 metres of rope dragging behind him wrapped around the lower leg, funny he says "that explains why it was going so slow" strange thing was though that the pot contained a live lobster, and he proclaimed it was all he'd caught all day.
Moral of all these stories is i suppose that if you do this and you use public slipways with these morons for long enough then eventually sods law dictates its goin gto happen to you at some point.