How does one "accidentally" overdose?

When you are use to a certain amount of a drug or if the drug is not as potent, like things were added to increase the quantity instead of quality of the actual drug, then receiving a purer form of the drug or a bigger dosage would cause accidental death because the one taking it umed it was the same as before....Like back in the seventies when cocaine was pure and today it is cut enough to not even be called cocaine. The real stuff would kill you if you are only tolerant of the stuff you were getting that had hardly any in it.