Michael Phelps has nothing to prove to anyone, but neither did Michael Jordan, Kelly Slater, or Gordie Howe. Still, these legends all got up off their laurels, un-retired, and tried to compete at a high level. Some succeeded. Others, not so much.
Why he hung it up: On November 7, 1991, Ervin "Magic" Johnson announced he was HIV positive and retired immediately.
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Why he returned: He realized retiring was a mistake, saying "If I knew what I know now, I wouldn't have retired."
His Success: Magic played in the 1992 All-Star Game and helped the Dream Team win a Gold Medal in Barcelona. He also played during the 1995-1996 season after coaching the Lakers for a couple seasons.