Many athletes refuse to wash a piece of gear out of superstition. The belief is that if they performed well in a game or match, they don't want to "wash the luck" out of the garment. Perhaps the most famous example is Serena Williams. According to a 2013 piece in the New York Times, Serena will wear the same socks for the entire duration of a tournament, refusing to have them washed if she is winning.
Repeatedly wearing sweaty, dirty clothing can lead to an increased risk of rashes or disease. But many athletes are willing to risk good hygiene if it means a little extra luck on their side. While it may sound crazy, research has shown following superstition can have a positive impact on performance.