Considering the man in this picture is just wearing a Yankees t-shit, I thought that maybe someone bumped into the photographer, resulting in a casual fan appearing in Mr. Mohorcic’s place.  This, however, is not the case.  Two amazing stories about Mr. Mohorcic from Wikipedia (though neither explain the t-shirt wearing):

1)  In a 1987 game against the Milwaukee Brewers, Mohorcic was accused of doctoring the baseball. Umpires found no evidence of wrongdoing at the time, but after the game Mohorcic complained of a sore throat, and was admitted to a hospital where it was discovered that he was suffering internal bleeding as a result of having Crohn’s disease and taking the pain reliever naproxen. It was erroneously reported by Peter Gammons that Mohorcic’s bleeding was caused by swallowing sandpaper.

2)  Mohorcic played the part of Angels starting pitcher Dave Spiewak in the 1988 movie The Naked Gun.