More specifically -

When ServiceProvider and metapath parameters are not present, it will complain that ServiceProvider is missing

When ServiceProvider is present, but metapath is not, kernel segfaults.

When metapath is present, but ServiceProvider is not, it will silently abort in the middle of the output. The result may look like this:

80
+Content-Type: text/xml; charset=utf-8
Status: 200 OK

<?xml version="1.0" encoding="UTF-8"?>

All of the above should yield a correct xml error message.