I was listening to a Human League compilation today at work. "Human" came on the middle of the album, and I realized that right there, at that specific point in their career, the band decided to make a song that would spend the next thirty years playing in bank lobbys and Burger King dining rooms. This song was their Ralph Wiggum moment: if you watch the video, you can actually pin-point the second when this band stopped being awesome.
Yesterday I read this excellent Chuck Klosterman interview with Noel Gallagher, where Noel touches on similar themes about having "lost it" from a songwriting perspective in his own career. It also reminded me of sports; there's a classic image of Mickey Mantle tossing his batting helmet late in his career when he was in one of those terrible seasons that ended up costing him his lifetime .300 average. That image was the moment you knew it was over for the Mick.
What are some of the other classic moments when a good act definitively stopped being good? It could either be in the context of a gradual decline or an epic collapse, but what singular moment stands out as the point when you knew that the act had irredeemably lost it?
Edit: I put this in the misc. lounge because the topic lends itself to all sorts of arenas beyond music.