I know people can sue medical doctors (and I assume p-docs) for malpractice--there are lawyers that specialize in that who advertise on TV a bunch (like for medical mistakes or drugs that caused permanent side effects). I'm not sure about therapists though--I imagine that's a fuzzier area and harder to prove that they breached a contract or caused harm rather than good. (I mean, assuming it wasn't a case where they were double-billing you or something).