When I was regularly dosing people, I always aimed for a breakthrough dose, regardless of pretty much everything, including psychedelic experience.
Out of dosing somewhere between 30-40 people I only had two people who had rough experiences. One of them was someone who, in hindsight, I knew I shouldn't have dosed (for a myriad of reasons), but did anyways at his insistence. After taking his first (fairly small) hit, he panicked and refused to take anymore, choosing instead to attempt to fight the trip the whole way through. Needless to say, he had a rough low dose experience.
The other person who I witnessed have a rough trip took a larger hit and held it in for longer than I've ever seen anyone hold a dose. After finally exhaling his dose, he lay down quietly on the bed for about one minute. Suddenly, his eyes snapped open and he began screaming and thrashing. I layed across him in an attempt to keep him from hurting himself/breaking things/trashing my room.
I was at least his weight if not bigger and was barely able to impede his actions. We wound up getting bucked onto the floor where he resumed screaming and milder thrashing for some minutes. I lay on top of him and spoke soothingly into his ear, trying to reassure him (if there was even a chance of him hearing) that everything was going to be ok and that he was safe. He soon quieted down and went slack, whereupon he went still and was gone for the next ~25 minutes. At around 30ish minutes or so into the experience, he opened his eyes, muttered incoherently and crawled onto the bed, where he curled up under some blankets and went still again.
He occasionally talked to himself or made sounds over the next 30-45 minutes before emerging from under the blanket and grinning uncertainly at me. He said thank you a number of times and stated that he had only a patchy memory of what had happened. He was shocked to hear the recounting of what had happened while he'd been elsewhere. He left in an awed and thankful manner, but it was certainly a very rough experience.
Everyone else that I dosed to breakthrough followed the same general pattern of taking their hit(s), leaning back/closing their eyes, and grinning for a long period of time, before sometimes occasionally opening their eyes/talking for a brief period and then closing their eyes/resuming the tail end of their experience and then coming down. The vast majority of people, including several individuals who had no prior psychedelic experience had beautiful and/or transcendental experiences.