Nintendo has stated that it's working on a patch, and confirmed modes on both the Wii U and 3DS versions that could prompt this glitch - full details are right here.

Original Article:

The arrival of Mewtwo in the latest Super Smash Bros. titles today - for Club Nintendo members, that is - has certainly prompted plenty of players to fire up the games and get into some brawls. Some unfortunate players, however, appear to have triggered a rather unlucky glitch that's preventing them from playing Super Smash Bros. for Wii U online.

In a problem being reported by multiple gamers, it seems that if you've cleared 10-Man Smash with all characters and then beat it with Mewtwo, it's possible to trigger a corruption of save data. It resets the overall GSP for that mini-game to zero, and when trying to play online those players are simply getting the following message.

You have been disconnected because of irregular save data.

In most cases these players have met the criteria above - they've cleared 10-Man Smash with all characters and then done so with Mewtwo. The issue seems to be related to that particular mini-game for the majority, with logic suggesting that a piece of faulty - or old - code is wrongly interpreting the Mewtwo data as some kind of attempted cheating, or it could be straight-up data corruption.

There are multiple reports of this on Miiverse, SmashBoards and Nintendo's own Tech Forums. One user did report that they were on the phone with Nintendo customer support for 40 minutes but were unable to resolve the problem, and they've been promised a follow-up call. It appears that this issue is only affecting the Wii U version of the game, not the Nintendo 3DS version.

Until we know the full score, or until Nintendo issues an update to fix the issue, it may be a good idea to avoid 10-Man Smash with Mewtwo. In fact, to play it particularly safe perhaps stay away from the Multi-Man Smash events as a whole.

Do you know anyone who's been affected by this, or are you one of the unlucky victims to date?

With thanks to Kazuo Yokoyama for the tip.