Originally from Siliconera at

Capcom have come out with a strongly worded notice for players on their official
website, warning them not to tamper with Monster Hunter 4’s game save data.
They warn that doing so could possibly brick the entire game itself, rendering
the copy unusable.

While we don’t know the full extent of the hack’s capabilities, game blog
Hachima does have a video that shows off some of what the hack offers—including
a non-stop rotating weapons icon set among other hints that all is not quite
right with this video. The ending of the video is kinda like the Hitler meme
that makes its rounds, overlaying an actual movie scene with its own comments
below it basically expressing gamer’s anger at the hack.

It seems some hackers have broken into the game’s underlying systems, giving
themselves either ultra-rare and otherwise impossible-to-obtain weapons and
armor or changing up what sort of monsters can be hunted or items dropped in
guild quests. It also looks like they’ve been able to adjust game AI to
basically ignore players, and possibly even spawn rare monsters in locales of
their choosing.

Capcom specifically states that gamers who receive strange guild quests or
Felynes from StreetPass correspondences should immediately delete them due   
to the risk of tampered save files corrupting your own. Players who are found   
to have fiddled with their save files at Capcom’s Monster Hunter Festa in-game 
events will be banned from it and all future events.

Considering how many copies the game has sold it’s hard to know how widespread
the hack is and how many players could be in on it. Just say no, it takes the
fun out of the game anyway, and why would you want to ruin it when you can
eventually get those super-cool Thunder toys?

I honestly thought at first that 3DS games can't be hacked. They can't as of yet, but the savefiles can. The digital version of the game saves on the SD card leading to vulnerabilities.