No you cannot overdose on marijuana. It is just not possible and I have never heard of anyone overdosing on it. I have heard of people getting dehydrating from smoking too much, but that is about as serious as I have heard. It's a very mild drug compared to other drugs.
It is absolutely impossible to overdose on marijuana alone. Anything that suggests otherwise is inane propaganda and should be done away with. If there was ANY case where someone actually did overdose on marijuana it would be seen everywhere, but yet there isn't. There is plenty of empirical evidence to support this claim too.
There is no recorded cases of anyone ever reaching lethal toxicity from smoking or ingesting marijuana by any other other means. One would have to ingest the all of THC content in about 1,500 lbs of marijuana in an extremely short period of time (say, 15 minutes) to reach toxic levels in the body.
No one has ever died from an overdose on marijuana (cannabis) itself, and that is a proven fact. The high from marijuana has a stopping point of sorts- there comes a point where you can't get any higher even if you keep smoking. The only side effects one will feel from too much marijuana is being lazy or more groggy the next day. If someone unknowingly ingests marijuana that is laced with a harmful substance, then it is certainly possible to OD. However, the overdose would be a result of ingesting too much of the substance the marijuana was laced with, not the marijuana itself.