Game 5 of the 1976 NBA Finals: Celtics 128, Suns 126 (3OT). Start watching around the 90-minute mark to see things really get nuts:
The series featured recent Hall of Fame inductees Jo Jo White and coach Tommy Heinsohn. Boston took home the title in Phoenix two days later in Game 6 (below).
There's a great quote from Brent Musburger at the start of the broadcast, while discussing how Heinsohn collapsed after the epic Game 5 win: "[The doctor] has told him to stay calm, and he can stay out here and run the Celtics.