Wii U hit Super Mario Maker was recently ported to 3DS, and the handheld version of the game has an added emphasis on sharing in person. Unfortunately, this does not include sharing the cartridge with a friend. Some players have complained about their save data being erased, prompting Nintendo to release an official explanation.
As it turns out, lending your game to a friend to play in their 3DS will actually erase all of your save data. This, Nintendo explains, is due to the fact that Super Mario Maker for Nintendo 3DS stores save data on both the cartridge itself and the SD card. Both files are required to continue playing, so if the cartridge is put into a second 3DS, the save data on the cartridge is deleted.