It can lead to psychosis. It happened to my partner. He had the mother of all bad trips that led him to hallucinate that his loved one who was tripping with him was a murderer who had poisoned him and was trying to finish him off. He shot her believing it was self defence. She died. Now he's in prison for manslaughter. For years after the bad trip, he had constant hallucinations and delusions. Through medication and Internal Family Systems Therapy he has mostly recovered but he will never be the same again.

It's rare but it does happen.