Three of the worst calls in MLB history that I remember, that changed the entire complexion of each series...
Rich Garcia awarding Derek Jeter a homerun when Jeffry Maier clearly took the ball off the field of play, Baltimore was on the brink of going up 2-0 vs NYY, and the series was coming back to Balto for 3 games, instead series tied and Balto ended up losing the next 3 games!
Denkinger game, the Cardinals and Jaquin Andujar , complete meltdown 11-0 game 7 loss.
The White Sox, to a lesser extent , because they had not lead the game...but did never recover , losing 3 straight.