I've never had a bad high with just pot alone. The closest I've come is one time where I felt kind of sick after smoking a ridiculously large amount, but I'd been a bit headachey beforehand, so really it was my own fault. I should have just done a bit and gone to sleep instead of hanging around.

I had an awful high once with pot laced with shrooms, though. Hallucinating + being effectively locked in a nearly empty, white room about the size of a closet = baaaad trip.