Okay, the San Francisco Giants lead the Detroit Tigers 2-0 in the 2012 World Series; but the Detroit Tigers came back from a more dire situation in the 1968 World Series. Here's what happened:
Game 4 was a blowout when Detroit lost to the St. Louis Cardinals, 1-10. The situation was bleak because the Tigers were down 1-3 games in the series and had only one more game in Tigers Stadium before heading back to St. Louis for Games 6 and 7.
Not only did Detroit win Game 5, they won Game 6 decisively, 13-1. Turns out, St. Louis couldn't find a reliever that could do the job. In fact, the Cardinals cycled seven pitchers to the mound during the course of Game 6, while Tigers' pitcher Denny McLain kept the Cardinals down to one run. The blowout was apparently enough of a psychological boost for the Tigers to win Game 7 and take the World Championship.