If you buy a new MetroCard between now and February 3, take a look at the card itself: You may have received one of a million special Super Bowl–themed cards that are now circulating throughout the system. The cards, created to commemorate the 2014 event's New Jersey location, will be randomly distributed at 400 subway stations, and feature four different designs, one of which you can see above.
The commemorative cards are paid advertisements that were commissioned by the Super Bowl Host Committee, and went into circulation on January 6. This isn't the first time the MTA has placed an ad on the front of a MetroCard: The Gap was the first company to purchase space on the front of the transit pass, with cards that debuted in October 2012. (Those cards also functioned as a coupon for the retailer—too bad these new ones can't somehow help you avoid the bananas ticket prices for the game.) The MTA did not disclose how much the Super Bowl Host Committee paid for the promos.
One thing to keep in mind, if you're a superfan who really wants one of these keepsakes: The MTA produces around 5 million MetroCards per month, so there's a good chance you might not end up with one. But hey, you never know.