When Notch was developing 0x10c in 2012 he often livestreamed himself coding the game, which resulted in some fans copying the onscreen code to create a reverse engineered version of the game. Notch asked them to stop distributing it, but there was a brief period of time when it was being distributed, and some may still have it, particularly those who actually worked on it.
https://www.reddit.com/r/0x10c/comments/vdbkn/trillek_0x10c_room_editor_demonstration_by/
https://www.reddit.com/r/0x10c/comments/u5pxj/notch_some_clown_has_written_down_code_from_when/
https://www.reddit.com/r/0x10c/comments/u5kru/0x10c_upcoming_room_editor_release/