For me, it was the 1993 game @UCLA. We came in 4-0, ranked, and got the worst beatdown in BYU history to that point, IIRC. 68-14. I was in the band and we were getting jeered on the way out of the stadium. It was the beginning of a tailspin that somehow still resulted in BYU going to the Holiday Bowl (and nearly beating Ohio State). That was long before the dark independence era of toilet-tier bowls.