Someone recently showed me that he had used Action Replay to gain impossible stats for custom fighters in Super Smash Bros for 3DS. This prevented him from uploading characters to the Wii U version of the game (the Wii U said the data couldn't be imported because the game file was corrupted). Apparently people also use similar hacks to get impossibly strong weapons in Kid Icarus Uprising, and I imagine other games with online multiplayer and/or Streetpass capabilities are affected.
Can players who have hacked game files such as these Streetpass other players in those games? I'm concerned about getting corrupted data from such players via Streetpass.
Also, can hacked game files be used in online play? It's obviously unfair if this is possible. Is it determined by each game, or do Nintendo's servers check when you try to play online?