Last week, I was working with a new client on an issue involving MS Sharepoint Services and Sharepoint Portal Server. This was actually a business application written on top of Sharepoint - not your typical Intranet or Extranet implementation. As a result, there were many pages in the Sharepoint sites where there were 4-5 custom Web Parts with interdependencies. We were using Session variables to contain certain process and transition states. And there is the rub... we were losing the contents of those Session variables on postback most every time.
After many hours of questioning, debugging, and Googling the issue to death, we got on the phone with Microsoft. It turns out that the Sharepoint server in question had an underscore "_" in the Windows 2003 server name, something like "sharepoint_test". According to Microsoft, that's a known issue. Luckily for us, the server didn't have something on it which depended on the server name, like SQL Server. (I know it's possible to change it, but hey, it's a royal pain in the neck!) Once we changed the server name, all was good again.
Let's be careful out there!