Continuity mistake: Adding to Kelly Leak's homerun in his first at bat shown in the movie (great scene with the ball hit during practice and clears the fence during a game), the celebration at home plate does appear to indicate a game winning homerun (a walk-off no less). However, as the ball clears the fence you can see the score is 0-0 in the top of the third. (00:57:52)
Suggested correction: It was probably just excitement that someone on the team finally hit a home run. This was a bumbling team (remember their first game they didn't get a chance to even bat) and were lacking hits and runs until now. Most likely a confidence booster for the team.