How does string.format handle undefined behavior?

[Halalaluyafail3 <luigighiron@...>]

There was a discussion some time ago about undefined behavior with the
next function, which made me think about other potential places for
undefined behavior.

The string.format function definition states: "Returns a formatted
version of its variable number of arguments following the description
given in its first argument, which must be a string. The format string
follows the same rules as the ISO C function sprintf. The only
differences are that the conversion specifiers and modifiers *, h, L,
l, and n are not supported and that there is an extra specifier, q."
Considering it is stated that it follows the same rules as the ISO C
function sprintf, presumably it has the same undefined behavior that
the ISO C function sprintf does, using the ISO C definition of
undefined behavior. I don't think this is intended, but from looking
at the source for string.format it does seem to accept format strings
which invoke undefined behavior and calls sprintf using them as the
format string.

What is the intended result of passing a format string to
string.format which invokes undefined behavior according to the ISO C
definition of sprintf?