gistfile1.txt

LUA:

-- prints "nil"
print(undefined_var)

AS3:

// doesn't even compile, gives you "Access of undefined property" compiler error.
trace(undefined_var);

Note that for AS3 I'm NOT talking about an existing variable whose value is "undefined", which would be e.g.:

var x:* = undefined;

I'm talking about a variable that has never even been defined at all, i.e. no 'var' or 'const' statement anywhere.