From Mental Floss:
The Promotion: Disco Demolition Night. White Sox fans were encouraged to bring old disco records to the park in exchange for a reduced admission price of 98 cents. The records were to be destroyed in between the two games of a doubleheader against the Detroit Tigers.
What went wrong: Believe it or not, a lot of people wanted to see disco records destroyed. 50,000 people showed up at the gates and many who were turned away at the gate tried to climb the walls of the stadium to get in. The crowd, who were reportedly heavily under the influence, soon realized that records could double as Frisbees, which naturally led to fans throwing firecrackers and drinks. When the demolition moment came, the explosion was bigger than expected and ended up ripping a hole in the outfield grass. Thousands of fans ran onto the field to join the mayhem, burning banners and throwing objects. The batting cages were even destroyed in the riot.