Barry Bonds, 1992
Yup. The big one. Imagine the scope of the steroid scandal if Bonds hadn't followed his godfather Willie Mays and father Bobby to San Francisco and joined the Giants in the 1992-93 offseason, but instead had wound up shaking hands with the Yankees? Pure chaos. He probably would've ended up banned for life (or eventually welcomed onto ESPN's Sunday Night Baseball broadcast).
According to Bonds (ironically as related on ESPN's KayRod Cast this past fall), the Yankees offered him a four-year, $36 million offer, but put a quick time limit on it. As Bonds grabbed lunch and tried to process the fact that the Yankees front office was hassling him, his agent called and told him the Giants had just outbid New York anyway. Bonds preferred to go home, and the rest was history.
But who knows? If the Yankees have a higher initial offer or don't push Bonds to shake their hands, maybe he chooses them before the Giants ever call? Maybe Maddux agrees, too (or they make a formal offer)? Maybe the '90s Yankees have the greatest left-handed slugger of all time taking aim at the short porch from 1993-1996, while Bernie Williams cowers in Boston?
Either way, history was changed that offseason, and it's amazing the Yankees didn't knock their championship train off the tracks right then and there.