In the film, Ross is mentioned as having played a game against the Houston Astros during his first stint with the Brewers. But at the time the fictional game was played, the Brewers were in the American League and there was no regular season interleague play, so the Brewers and Astros would not have played each other.
Tom Arnold mentions a midget who "played for the White Sox." Eddie Gaedel was a 3'7" man who played in one game for the St. Louis Browns in 1951. At the time, the Browns were owned by Bill Veeck, who later owned the White Sox.
T-Rex says he played ball at Fairmount Park in Philadelphia as a kid. Fairmount Park is the city's municipal park system, named after the original Fairmount Park, which is now East Fairmount Park and West Fairmount Park. T-Rex should have said he played at East Fairmount Park or West Fairmount Park.
When the Brewers are playing Astros as they say Gervis is coming to bat it shows a shot of the scoreboard and it says the Astros are leading 5-1 but if you look they scored 3 in the first and 1 in the second and none in the third. Clearly it is the top of the 4th because the Astros are up to bat.
Right after the Ice Cream song scene, a montage of baseball clips is set to Rob Base's song 'It Takes Two'. During the montage, the Brewers are shown playing the Chicago Cubs at Wrigley Field. The Brewers are in the 3rd base dugout, while the Cubs are in the 1st base dugout. The Cubs dugout is actually the 3rd base dugout; any visiting team is in the 1st base dugout.