Working with IIS on a particular app, we had a situation where the behavior of the web server seemed to be inconsistent with how we had configured it. We had configured it to allow Authenticated access based on Integrated Windows authentication only, no anonymous access. When we browsed to the page we wanted to view on the web server itself, the page came up fine. But when we tried to browse to the page from another box, we were required to authenticate. This was not expected since it should have been using the Integrated Windows authentication to log in transparently.
After comparing it with another server that worked and seeing that most of the GUI configuration looked identical, I started googling and found this article that answered the problem for the most part. The only difference we found was even the order of the different NTAuthentication providers matters - the working server had "NTLM,Negotiate"; the server that didn't work was set up as "Negotiate,NTLM". Changing the broken server to look like the other server fixed our problem.
Our infrastructure teams has been doing some changes and moved the configurations for the email servers a bit. Unfortunately, this wasn't transparent to my Nagios/GroundworkOpensource installation (since I was lazy and hadn't reconfigured the email 'from' address for host). What ended up happening was all the Nagios emails ended up getting sent outside the internal network (because the domain name was still 'localhost.localdomain') and getting caught by the public antispamm appliance we have. I ended up switching notifications to use 'service-notify-by-sendemail' instead of 'service-notify-by-email' and then overriding the host in the command to point to our local email server. And that worked. Similar to this post on the groundwork forum.