What
A variable holding a LuaSurface does not point to that unique created surface, but instead seems to point to whichever surface has that specific index. For example if a the variable's surface is destroyed, and a new different surface created later, it might have the same index. And in that case the original variable will point to the newer surface.This can easily be seen using the following commands (on a fresh vanilla save). Create initial surface
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/c foo = game.create_surface("foo")
/c game.print("Foo: "..(foo.valid and foo.name.." "..foo.index or "invalid"))
Now delete the surface
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/c game.delete_surface(foo)
/c game.print("Foo: "..(foo.valid and foo.name.." "..foo.index or "invalid"))
Now for the unintuitive part, create what the modder might believe is a different surface
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/c bar = game.create_surface("bar")
/c game.print("Foo: "..(foo.valid and foo.name.." "..foo.index or "invalid"))
Why
Modders can incorrectly assume (like I did
) that a LuaSurface acts just like a LuaEntity, and when destroyed it will remain invalid. And this can create hard to track down cache-invalidation bugs (like one I caused 
). I know this is probably due to how the underlying C++ surface implementation works and is probably not a bug. Might just be useful to explicitly state it in the docs, especially since mods are really starting to embrace the multiple surfaces.
