While overdose from MDMA itself is rare, many more toxic substances are often sold as ecstasy, and overdose or other adverse reaction to adulterants is not uncommon. MDMA appears to be one of the most commonly adulterated drugs.
Many legal pills such as aspirin, paracetamol (acetaminophen), or even canine heartworm tablets have had the letter E scratched into them and been sold as ecstasy, for enormous profit. (This is a minor plot device in the movie Go.) Such false labeling can have deadly results, as a significant number of people are allergic to aspirin. Paracetamol is fairly hepatotoxic and can cause significant liver damage or death if taken in large doses, as might happen to a person taking four or five tablets in quick succession, thinking the tablets to be ecstasy.