when do programs set metatable?

[Roberto Ierusalimschy <roberto@...>]

More often than not, programs set the metatable of a userdata right
after creating the userdata. Does anyone know of relevant counterexamples
to this pattern?

More specifically: if Lua puts a cost on setting the metatable of an
object long after its creation (the longer the more expensive), do you
know of programs that would be hurt?

Yet more specifically, I am interested only in cases where the metatable
has a __gc metamethod.

-- Roberto