Happens all the time to regular people who are daily drinkers and are then given a narcotic. They vastly underestimate the warnings and carry on with their "normal" activities. Respiratory arrest can sneak up on them. So can choking on their own vomit.
In retrospect, it probably was an accident, but not in the sense of taking small amounts of both drugs. Someone just tried to get a little too high, and miscalculated.