Everyone here seems to have a different view of what an overdose is. Some people are taking it as the same as a heroin overdose, some aren't.
Lets try to define what the OP means by overdose before we get into too much more depth on this concept. We obviously have found that overdosing in the sense of drugs like heroin, or even prescription drugs is nearly impossible, or at least extremely, EXTREMELY difficult.
Remember also that for those who have added their experiences with "overdosing" on marijuana, that there is always the possibility that what you had was laced with another drug, like PCP or LSD, which can make consuming it in any fashion fatal if you consume too much, but only because of what it is laced with. This is part of the problem with buying marijuana off of the street from dealers, and is a big reason why supporters of the legalization of marijuana would like to have it legalized. You know what you're buying, and you know it's safe (or at least safer, depending on your POV on marijuana).