The Grand Slam Single is a reference to the hit that ended Game 5 of the 1999 National League Championship Series between the New York Mets and one of their rivals, Atlanta Braves. The game was played on October 17, 1999 at Shea Stadium.
The play 
The game was tied 2–2, going into the top of the 15th inning, until Mets pitcher Octavio Dotel gave up an RBI triple to Keith Lockhart, giving the Braves a 3–2 lead. In the bottom of the 15th inning, the Mets loaded the bases against Braves relief pitcher Kevin McGlinchy. Mets catcher Todd Pratt drew a bases loaded walk, tying the score 3–3.
The next batter was Mets third baseman Robin Ventura. Ventura crushed the 2–1 pitch over the wall in right-center for an ostensible grand slam, winning the game for the Mets and driving the Mets players and fans into a frenzied celebration. Ventura, however, never reached second base as Todd Pratt, the runner who was on first, picked up Ventura in celebration. Subsequently, Ventura was mobbed by his teammates, never finishing his trot around the bases. Because he failed to touch all four bases, the hit was officially scored a single. Roger Cedeño, the runner on third at the time, was ruled the only runner to have crossed home plate before the on-field celebration began and the Mets were awarded a 4-3 victory. Thus, Ventura was only credited with a single and one RBI. As a result, there had never been an official walk-off grand slam in MLB postseason history until Nelson Cruz hit one to allow the Texas Rangers to beat the Detroit Tigers 7–3 in Game 2 of the American League Championship Series on October 10, 2011.
Defining the "single" 
Sports books in Las Vegas were put into an unusual situation with the "single" as a final score of 7–3 (the score that would have been had Ventura completed his trip around the bases) meant the game would have gone "over" the over/under line, which was 7½ . However, the final score actually put the game "under", meaning that many bettors that should have received payouts did not.
The play remains as one of the most memorable moments in Mets postseason history. Orel Hershiser, who played on the 1999 Mets remarked, "It will be right up there with Kirk Gibson's home run (Hershiser was a teammate of Gibson with the Los Angeles Dodgers during their championship season of 1988), Carlton Fisk, Bucky Dent. This one will be on that tape with them." However, the Mets went on to lose the series to the Braves.
Other instances of "grand slam singles" 
According to Baseball-Reference.com, there have been at least two other instances of "grand slam singles." Both occurred when a batter hit a grand slam but subsequently passed the runner ahead of them on the base paths, which according to the rules of Major League Baseball causes the runner who passes his teammate to be called out. This happened on July 9, 1970, when Dalton Jones of the Detroit Tigers passed teammate Don Wert in a game against the Boston Red Sox, leaving him with a 3-RBI single.
It also occurred on July 4, 1976, when Tim McCarver of the Philadelphia Phillies passed teammate Garry Maddox during a 10–5 win in the first game of a doubleheader against the Pittsburgh Pirates, their in-state rivals, in Pittsburgh, leaving him with a 3-RBI single. In both cases, the other three runs still counted because only the player who passes his teammate is called out. The three baserunners are able to score. Both of these hits took place with fewer than two outs.