OK, I found this story from an interview Red gave to Bob Costas in 1991.
Was there ever one time ? Yes, just once the cigar was ignited a bit prematurely. Sportscaster Bob Costas asked Auerbach in 1991 if he ever lit up and lost the game. Without revealing the team the Celtics were playing, Auerbach told the tale .........
"We were three points ahead with 10 seconds to go (no three-point shot then) and they had the ball, Auerbach recounted. So, I lit up the cigar and said let em score. Don't touch them. Sure enough, Tom Heinsohn decides to block a shot. He hits the guy and the ball goes in. They make the free throw. They tie it up. We came back out and won in overtime. We didn't blow the game, but they tied it up, Auerbach recalled. I could have killed Tommy."