The Most Dangerous Match

Although Clayton is one of the best chess players in the world, during his simultaneous exhibition, one of the people he is playing beats him after playing only two moves, a losing combination known as the "Fool's Mate" and notorious for being the fastest possible way for a player to be checkmated. This, presumably, is meant to demonstrate how much Columbo is unnerving him.

The list that Clayton is re-creating from memory shows digitalis and digoxin as the first two items on the list. These two are the same thing, and if a person was to take double a life-saving dose, it would be fatal.