Fatal marijuana overdose in humans does not exist. The scarcity of cannabinoid receptors in the medullary nuclei is largely the reason.
Nevertheless, heavy doses can produce unpleasant reactions and, in some rare cases, major doses can result in panic reactions characterized by anxiety, paranoia, self-consciousness, loss of self-control, and disorientation. These reactions subside with a few hours with no medical treatment.
Such panic reactions will most likely to occur in novice users who have tried excessive doses. First-time users should be especially careful. Overdoses are less likely with inhaled marijuana than with oral ingestion, because smokers sense instantly when they have had enough or when the psychoactive content of the drug is too high.
Oral doses are harder to quantify because you can eat several “doses” of brownies and not feel any different until an hour or two later.