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: After testing positive for marijuana for the second time, Dolphins running back Ricky Williams received a $650,000 fine and a four-game suspension. He opted to leave the game NFL.
Why he returned: Ricky's reasons for doing anything are unclear, but after an abysmal first season back in the league and a little bit of time in Canada, he settled down and became a viable NFL player again.
His Success: He continued to pile up yards, fail drug tests, and generally be one of the league's most interesting and enigmatic characters before retiring for good in 2012.
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