Kramer and Newman went to a baseball game during the summer of 1987. They sat in the right field bleachers and they heckled Roger McDowell, who would spit on them after the game, while Keith Hernandez watched. According to Newman, the date was June 14, 23 years ago today.
Of course, with the Internet, we can quickly disprove Newman's version of events. On that day a Keith Hernandez error did not open the door for a five-run Phillies' 9th, costing the Mets the game. The Mets were actually playing in Pittsburgh on June 14, 1987 and won, 7-3. Hernandez went 2-for-4 with a homer and a double.
The two-part episode that portrayed this incident, "The Boyfriend," aired on February 12, 1992, during Seinfeld's third season. So in the nearly five years between the incident taking place and Newman telling the story, it's understandable that Newman might have confused some details. However, when you combine that with the fact that he and Kramer had been holding a grudge against the wrong person for half a decade, you start to wonder what else they might have gotten wrong during the show's run. No wonder Jerry doesn't like Newman.