you're not characterizing overdose correctly. it's often people coming from pharmaceuticals and not knowing what they're doing, or not knowing the potency or contents of their drugs.
The majority of overdose deaths occur after a period of abstinence (getting clean, running out, etc.) followed by using the same amount (or more in the case of withdrawal) of the drug before the cessation. It can take as little as 48 hours for your tolerance to drop by a meaningful margin.
My ex-girlfriend died in September of this year from a heroin overdose. She was in the process of getting clean and then relapsed.
Another reason for a large amount of overdoses is drug mixing - people taking oxy and then xanax and then they're dead. But I think your comment and the guy above you both refute hapless's claim that most people who die from an OD are simply misjudging their own doses. In truth, I believe very few people misjudge their own doses. Drug addicts are notoriously good at knowing the exact amount to get them high. Why? Because too much is a waste (they try to extend their supply as long as possible) and too little won't have the desired effect.
In addition to your reason for ODing (use after a period of abstinence), the recent rash of ODs in my state are because dealers & suppliers are cutting their dope with fetanyl and other stuff that the drug user isn't/can't account for.
One of the more common "mixes" that also happens to be particularly dangerous is drinking EtOH while taking an opiate (or other depressants). Unfortunately, as beer or liquor holds a different cultural status, it isn't always listed with other drugs.
If there is any opiate involved, it will invariably be written up as a "oxy/heroin overdose" even if the main problem was the 10oz of liquor consumed the same evening.
My ex-girlfriend died in September of this year from a heroin overdose. She was in the process of getting clean and then relapsed.
http://www.businessinsider.com.au/philip-seymour-hoffman-ove...
https://www.thefix.com/content/high-risk-relapse-how-going-b...