I stepped on a stone pressure plate next to a noteblock. Instead of making a sound, the game gave an error. When the game was restarted, parts of the building and the original biome (desert), with a perfectly square format of 16x16, were replaced by a lower positioned grass biome. This 16x16 area was on the exact place I was when the error occured. After I restarted, I was in the middle of this strange new grass biome, suddenly flying.