A problem has arisen with
Smash for the Wii U that's causing some users to encounter error codes, leaving their games and sometimes their consoles unplayable. The most common error codes are 160-0103 when the game is booted up, and 160-1710 on the Data Management page. Many are suspicious that this error is related to an update, but there's no official word yet. These errors can sometimes leave a console bricked, especially after deleting an update, so proceed with caution. Affected users who have contacted Nintendo are being directed to format the system (which can leave it bricked), or mail the console to Nintendo.
The issue is not currently very widespread, but be careful out there, Wii U owners.
UPDATE: Internet sleuthing has revealed that it isn't necessarily Smash's fault; it's been happening with a variety of games.
UPDATE 2: Nintendo is investigating the issue, but maintains that it's only affecting a small percentage of consumers.