Originally from Siliconera at http://www.siliconera.com/2013/10/08/dont-hack-save-file-monster-hunter-4-capcoms-watching/
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.