I recently had this weird experience where I started a new file on my copy of Black. Since I have a full pokedex on my current file I left my 3DS on and closed it when I wasn't playing. By the time a reached Desert Resort my pokemon just stopped gaining experience and there stats seemed to be hinged.

I restarted the game and it said it could read any save data so I blew into the cartridge and it was fine again.

It sounds like the anti-piracy measures of the game are failing. Try playing the game in different systems.

I remember when that sort of thing happened when people tried getting Black and White 2 as early as possible through certain methods. What's weird though is that something like that happened on your actual cartridge. Oh well, gotta love when technology screws up.

It's fine now that I've reset it but it was really weird, I've done this many times before and this has never happened.

I saw the title of this thread at the top of the forum's home page and briefly thought it was news for X and Y. It would certainly be a unique direction for Game Freak to take for the Pokemon franchise.

Speaking of cartridge contacts, I'd advise that you get both the card and system contacts checked at this point. It's probably worn down so much that strange things might be happening with regards to data transfer to and from the system and card.