It was said that Guest 666 would use admin commands and chat when in-game, which was something normal guests couldn't do. However do to the myth being fake it is very likely these are all just speculations and rumors.

Unlike the other guests, Guest 666 was supposedly capable of using chat to view messages and talk themselves.

Guest 666 is an old Roblox myth and creepypasta who was rumored to have admin commands in any games they joined (even ones without admin built in.)