“Off by One” in the context of mentalism refers to a strategic “out” or contingency method used when a prediction appears to be slightly incorrect, but the performer turns this near miss into a surprising and satisfying conclusion. Popularized by Kenton Knepper in his “Kolossal Killer” routine, this principle allows the mentalist to predict any playing card named by a spectator while significantly reducing the number of physical outcomes needed. Instead of requiring 52 separate predictions for each card, the performer relies on a smaller set of cards, each marked with the phrase “Off by one!” on the back. In practice, if a spectator names a card (e.g., the 8 of Spades), and the performer has that card concealed, they reveal it directly. However, if the named card is adjacent in value (e.g., 7 or 9 of Spades), the performer reveals the closest match (e.g., 8 of Spades) and then uses the phrase “Off by one!” as a twist. This approach transforms an apparent failure into an intentional and clever outcome, maintaining the illusion of foresight. The “Off by One” technique has since become a widely recognized tool among mentalists, offering flexibility and reducing preparation complexity while still delivering impactful results. It exemplifies how mentalists can use creative scripting to turn limitations into strengths, ensuring that every performance feels intentional and astonishing.