Many thanks to Mars the Frog for this information:
In his own words:
'It recently came to my attention that some hackers have been making their own DLC items (downloadable content). The known ones are the shiny blue pikmin hat, and two lightsabers (made from golden / silver axes) - red and blue ones. The exact consequences of owning such fan-made hacked items are unknown, but there are some people whom believe the damage they may cause to the game's data where DLC is concerned may be devastatingly bad. This is mainly due to the fact that the hackers are taking up codes within the game that are made specifically for real DLC items from Nintendo - like the Top and Red Pikmin hat. Others think that the real DLC items - when downloaded - will simply overwrite that data and all will be well.
The bottom line is - no one knows for sure wether or not it's safe to accept these hacker items. Personally, I wouldn't take any chances... but even that's difficult, due to the easy way of obtaining item data in AC. All a normal player would have to do is have a visit from a hacker with one of these items... and then the data would be implanted into his or her game... In that way, it's sort of like a virus... These hackers though, at least the ones behind the lightsabers and shiny blue pikmin hat, aren't doing this for malicious fun - they're just risk takers that enjoy screwing with the game and sharing the fruits of their coding labour.'
As you can see, you do need to be careful where visits from other players are concerned. Even without malicious intent, your game could be compromised by a hacker who visits your Village due to the information data HIS/HER game.
On a wistful note... who wouldn't want a 'shining lightsabre'??? But it isn't worth the risk of ruining the entire game. The problem for a non-hacker, like myself, is that I would not be able to manipulate my game data in order to 'fix' any code problems that occurred from hacked items.
I would advise players only to exchange Friend Codes with other players whose games have not been compromised by hacker data unless they are capable of dealing with any potential side-effects themselves.
There were similar problems with respect to Wild World. A section in my Comprehensive Wild World Guide describes some of the more serious problems caused by hackers.