Apparently this happened after a lengthy rain delay at Wrigley Field on September 18 in a MLB game between the Pirates and the Cubs.  Here’s more from Deadspin.com:

 Taking advantage of what we imagine was lax ninth-inning security, this gentleman found his way right behind home plate and spent most of the final stanza engaged in all manner of obscene face-making—even getting himself into Comcast SportsNet Chicago’s slow-motion replays. Eventually security came and hauled him off