I posted a comment on /r/trees the other day about context dependent tolerance, and I was told I should post here about it as well to spread some knowledge.

Have you ever tried using a drug in a completely different setting than your usual? Then you've probably experienced context dependent tolerance to some degree before. Essentially you don't just build tolerance to what you're using you also build a context dependent tolerance which you can get rid of simply by changing your surroundings.

For trees obviously this isn't that big of a deal, you can get used to being stoned in your bedroom go out on a hike and get more stoned than normal, but with harder drugs there have been reported overdoses related to context dependent tolerance. This is a study on opiate users; just click on the "PDF(701k)" link at the top-left if you would like to read the full study, also page 497 is where the study starts talking about overdose. Essentially what they found is that simply by changing the setting heroin users could overdose on what would be a normal dose for them.