Why so many people here saying their Wii U died, I'm getting worried.
For most people, it seems to be an error code that leaves Smash unplayable, and eventually bricks the Wii U entirely. My Wii U dying appears to have been a freak accident because I never got the error code.
Correct me if I'm wrong, but the error code tends to appear after playing an online For Glory match. The best way to avoid it is to only play For Fun, or if you're really cautious, just don't play online at all.