Nintendo of Japan have confirmed that a fix for the various glitches plaguing Super Smash Bros. is in progress.
With Mewtwo, the psychic Pokémon star, having been downloaded by Club Nintendo members this week as DLC, it's caused some players to experience issues whilst attempting to connect to others online. A fair few players who attempt various Stadium/Solo modes - including 10-Man Smash - end up with corrupted save data.
Nintendo are advising those with the Mewtwo DLC to avoid playing through Classic, All-Star, Stadium, Trophy Rush, Smash Run until a patch has been released.
Have you experienced errors after downloading Mewtwo?