Access Denied for user 1

This was a head-scratcher, so I'll just put it here for posterity.

The user 1 account had "Allow private messages" unchecked (obviously), but still had "Show aggressive notifications" checked (since I presume that's the default). This resulted in a "You have one new message!" on every page, but when clicking resulted in an access denied.

Ideally, the logic for that drupal_set_message() would be smart enough to not show me that message if I can't access private messages, but since most development is happening in 5.x-2.x now, it didn't seem worth rolling a patch if it's going to change a lot.