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On July 15, 1994, the Cleveland Indians were at Comiskey Park to take on the Chicago White Sox.
With the teams locked in a tight race at the top of AL Central standings, White Sox manager Gene Lamont, acting on a tip, asked the home plate umpire to check the bat of Indians slugger Albert Belle, believing it to be corked.
Upon an initial examination, umpires were unable to find anything amiss, but pursuant to MLB rules, they confiscated the bat and had it stored in their their office at Comiskey Park. The bat would then be sent to MLB offices in New York for further testing.
During the game, someone from the Indians crawled into the umpire's office through the ceiling tiles, switching the bat with a bat from the collection of first baseman Paul Sorrento.
The umpires, upon returning to their office after the game, quickly discovered that the bat was not the one originally confiscated, noting pieces of broken ceiling tile on the floor as well. They immediately ordered the Indians to give the bat back.
The bat was found by MLB to have contained cork and suspended Belle for 10 games. That suspension was later appealed and dropped to just seven games.
Years later, it was revealed that pitcher Jason Grimsley was the one who had stealthily climbed through the ceiling to retrieve Belle's bat.
Ironically, Grimsley would be suspended himself for 50 games in 2006 for violating MLB's drug policy and would later appear in the Mitchell Report as well.
Reference: The Orioles Warehouse