This misunderstanding is one of the reasons why people think that hypnosis is dangerous. It's actually hard to say how this misunderstanding was conceived, but my guess is that some stage hypnotist wasn't able to wake a person up in front of an audience. One of the reasons that stage hypnotists can sometimes have trouble eliminating the trance is because hypnosis is such a pleasant state of mind. It would probably only happen to the inexperienced stage hypnotists, as there are many ways to terminate a trance.

But even if an unexperienced hypnotist cannot wake the subject up, it doesn't mean that they will stay in hypnosis. We enter the hypnotic state several times during the day, and we get out of it aswell. So if a hypnotic subject cannot wake up, the stage hypnotist can use some techniques, OR simply let the person stay in the trance. The subject will wake up eventually.