Today I tried upgrading a VS 2003 ASP.NET 1.1 application to VS 2005 ASP.NET 2.0 all went OK I had a play but I wanted to roll back from my sourcesafe system to the ASP.NET 1.1 as this is what the production server is using at present. Again this all went OK, or so I thought.
I then tried to debug my ASP.NET 1.1 verison in VS 2003 and nothing worked! In fact the site would not run in any way, and the debugger would not attach.
The issue turned out to be that the 2005 updgrade process changes all the extension mappings in the IIS virtual directory. You can edit these back by hand, but the easiest way is to delete the virtual directory and get it created again with a checkout from source control system.