* Dangerous combinations, like alcohol, can be a contributing factor.

* Black market drugs can have widely varying contents and potencies (rat poison and fentanyl).

* Tolerance changes have already been mentioned.

* Taking drugs in a consistent place and/or circumstances causes the body to prepare for consumption (like Pavlov's dogs salivating at the bell). Unfamiliar situations can catch the body off guard.

Varying potency. Dealers putting in fentanyl or stronger opiates to intentionally cause an overdose as marketing hype for their product (very common). Misjudging the dose of a known product. Tolerance. Using too much after some time stopped. Mixing opiates with other substances. I'm sure there are many more.

If you're using street opiates, it's going to happen at some point, guaranteed. I've yet to meet an opiate addict who has never overdosed and I doubt I ever will. Sticking to prescription opiates helps, but even then people overdose often.

Both, but for different reasons: The "overdoses" from prescription opiates are often acetaminophen vs. the opiate (no direct sources w/ numbers, but example: http://www.medscape.com/viewarticle/820039).

Whereas with illegal opiates it's often from a difference in purity from one purchase to the next. This is where a regulated drug market would alleviate that issue 100%.

Often overdoses happen when a former addict relapses. They'll take their old dose, not realizing that they have lost some of their tolerance.