Every stage hypnotist show I’ve seen usually winds down with the hypnotist soberly reminding everyone that that they should never, never be hypnotized by anyone who isn’t a “trained professional”.
Is there any truth to this? Is there any documented case of a hack-hypnotist causing serious trouble? If so, how?

Just my $.02, but it seems to me that anyone doing a stage act isn’t a trained professional anything…except, maybe, entertainer.

It has been shown that hypnotized individuals are more susceptible to suggestion in such a way as to possibly create false memories. If you have heard of “repressed memories” being brought forth by hypnosis, or “memories” of alien abductions being brought out this way, then you likely already know about such false memories. If you want more info, check out the Great Debates thread on whether or not the soul is a collection of our memories (it’s a bit down there on the list in that area) and see my message near the end of the thread on that topic. It has a bunch of links related to this.

“If your self-worth comes from being at the center of the Universe, then science will make you uncomfortable. If your self-worth comes from other things, then science won’t bother you.” – Carl Sagan