WebMD says no one ever overdosed on THC, in fact it is non toxic.
All smoke is bad and potentially lethal, but you can consume THC infused grain alcohol till you pass out then have someone give you more while you're unconscious.
No-one has ever overdosed from Cannibis, but there are THC concentrated medicines that people have overdosed on, although I believe they were also sick, which is probably why they died. Over doses are usually non-fatal in marijuana. [1]
The estimated lethal dose of intravenous dronabinol in humans is 30 mg/kg meaning lethality is unlikely. However, in a 170 pound adult male, or 77 kilos, that's 2.3 grams, which is less than a penny. That's the equivalent of about 6 cups of coffee worth of caffeine. It is objectively more lethal than many other drugs, but it's still relatively safe.