The Gengar Incident
Pokemon fans are quite familiar with the limited-time "mystery gifts" that are distributed by Nintendo over the internet, allowing people to connect online during a certain period of time to claim free in-game rewards. Before the widespread adoption of the internet, however, Nintendo would often set up events in real life locations that players would have to physically travel to in order to connect their console to a kiost of some sort.

One example of these mystery gift events was the Pokemon Platinum launch event, held at the Rockefeller Center's Nintendo World (Now just known as Nintendo New York), where pamphlets around the venue advertised the activities of the event. People were dressing up in cosplay, participating in various games and contests for prizes, getting the advertised mystery gift, as well as, of course, playing the brand new Pokemon Platinum. Fans were celebrating their favorite franchise and having a great time as a line of customers was going out the door and wrapping around the building. The event seemed to be going off without a hitch- That is until an earie announcement was delivered over the store's intercoms.
"Please note that an attendee has wirelessly distributed inappropriate content to Nintendo DS users in attendance. If you receive a Wonder Card titled "Pokemon Platinum Launch!" at today's event, please decline to wirelessly receive it or delete it immediately. Accepting this unauthorized content could corrupt Pokemon video game data."
Was there really a rogue fan going around distributing save data corrupting content? Well, sort of... While the claim of the content corrupting save data was basically just a bluff from the Nintendo World employees trying to scare attendees off from accepting the gift, it most certainly did contain innapropriate content.

The innapropriate content being distributed took the form of a Gengar with a racial slur for a name. I won't type it here, but it's a word you used to hear very frequently in Call of Duty lobbies.
The prankster didn't just stop at the name, giving the racist Pokemon the attacks "thief", "sucker punch", "sludge bomb", and "snatch" with the special ability "truant", had the Pokemon holding the item "black sludge", and set it to like spicy foods
Believe it or not, this content wasn't actually being distributed by Nintendo. In fact, the store didn't have any plans to release a mystery gift of any kind that day. It turns out that the fliers around the store advertising the free mystery gift was actually a fake, it was a doctored version of the real poster which didn't include the mention of any mystery gifts.
You probably have 2 questions on your mind:
1: How did this happen, and
2: Who could do such a thing?

The answer to question one is actually quite simple. The way that Pokemon mystery gifts were distributed before switching to sending them via online servers was through the use of special game cartridges with the sole purpose of distributing mystery gifts. These special cartridges would only be sent to retail locations holding mystery gift events, often including a big, red "not for resale" sticker on the front. Many of these "not for resale" cartridges do often end up being resold, however, with people usually buying these special cartridges to dump the contents of them on the internet for archival purposes. All it would to achieve this same effect would be to download the rom for one of these cartridges, edit the memory to your liking, and then load it up onto a flash cartridge of some sort, likely an R4. All this hypothetical person would have to do from there is launch the software on their DS and start walking around an area with lots of DS users.
The second question is a bit more difficult to answer. However, a certain user on the BulbaGarden forum seems to believe that the culprit is a user named Fenzo666, a user who reported both on the original event announcement (sharing the fake pamphlet) as well as the news of the pamphlet being faked.