From #1 prospect to nothing: Which player in the NFL was the biggest let-down/who had the saddest story?

If you don't get what I mean...let me just say Maurice Clarett A.K.A gotta get my goose on! This is what I mean by let down someone who didn't perform like everyone thought, or couldn't get the "thug" mentality out of their mind when they made it big. Who had the saddest story/ FLOP!