Lua has a type of value called "userdata" that is just an opaque reference to a C resource. There's no way for Lua to know how do a deep copy of that resource, if it's possible at all. Tables can also have "metatables" that modify how they function. It's not obvious whether a deep copy should also create a deep copy of the metatable (and its metatable, and so on) or whether the copy should share the original metatable. There's good arguments either way.