Vladimir Guerrero, 2003
The 2003-04 offseason set the Yankees up for ALCS failure in Boston almost as clearly as can be. It's basically a one-for-one comparison. Make bad move, end up short-handed. Choose wrong option, end with regret. Add Kevin Brown's contract, watch him shatter his own hand for fun.
Essentially, the only thing the Yankees did that offseason that worked was signing Gary Sheffield ... but even that move came with a tinge of regret, as they could've had Vladimir Guerrero. They could've had him for longer. He could've won the 2004 AL MVP in New York instead of Anaheim, potentially going head-to-head with Manny Ramirez in a memorable (and victorious!) postseason war.
Guerrero was reportedly Cashman's preference. George Steinbrenner? He'd long displayed a "fascination" with Sheffield, who was famously ex-Mets legend Dwight Gooden's nephew.
Sheff worked. Guerrero would've been a better idea. And who knows? Maybe Vlad Jr. isn't going on Yankee-hating diatribes if his father had won a ring or two in the Bronx. Maybe he chooses the Yankees instead of the Jays as an amateur. The ripple effects of this one are spooky. Even spookier than letting Andy Pettitte walk in free agency for some reason and relying on Javy Vázquez, Esteban Loaiza, Jon Lieber and Brown with the season on the line.