Balk

A balk is an illegal motion by the pitcher that's judged to deceive baserunners, and it's widely considered the most confusing call in baseball, since the rulebook lists more than a dozen distinct ways to commit one. The penalty is automatic: every runner advances one base, no exceptions, no reviews of intent. The most common balks involve a pitcher starting his delivery toward home and then stopping, or faking a throw to a base he's not legally allowed to fake toward. Umpires get real discretion here, which is exactly why the call generates so much argument. A pitcher can commit a technical balk a hundred times a season and never get called for it if nobody's paying close attention, then get nailed for the exact same motion in a tense ninth inning. Full rule breakdown at BaseballMonkey.