There has NEVER been a proven case of somebody overdosing on marijuana. Some people react differently from others and those who react negatively may experience panic/anxiety attacks, but not a single person has died from it. At least not by itself.
The one thing that separates it from all other drugs is the fact that it is NATURAL. When it's not regulated and dealers lace it with other things like angel dust to get people both hooked on it faster and give them a 'bigger/better high'... that's when overdoses happen and people die.