While there has not been any reported fatal case of marijuana
overdose (marijuana overdose being the primary cause of death, not relating to a mix of drugs or a prior medical condition), a person can most definitely produce very unpleasant and potentially serious side effects to the drug when consuming very high quantities of THC
at a time.
This is very possible when eating THC laced content (i.e brownies, cookies), rather than smoking it (you only get 10-15% of available THC when you smoke a blunt).
Side effects include increase in heart rate/ blood pressure, difficulty breathing, increase in body temperature, disorientation, nausea, decrease in muscle coordination.
Other more frightening, psychological/psychosomatic symptoms include hallucinations (i.e tunnel vision), paranoia or terror, panic attacks, loss of "self", loss of time perception, short-term memory loss, and slurred speech. It is important to seek medical attention if breathing becomes shallow and slow or if a person loses consciousness. Side effects also depend on the individual, the amount consumed, and an individual's tolerance level.