Randall Cunningham
Cunningham had a decent career with the Vikings throwing to Randy Moss. But the team wasted a great opportunity to win a Super Bowl and Daunte Culpepper replaced him. Then he bounced around the league as a backup quarterback with mixed success (via Bleacher Report).
Some people think that he held on for too long and should have quit earlier. He had a decent impact in Dallas as he stepped in for the injured Troy Aikman. But there’s nothing else to say about his later adventures with the Ravens. In short, he could have quit and saved himself from the physical trauma.