Recently I was working on upgrading this site to from ASP.Net 2.0 to ASP.Net 4.0. As a first quick pass, I followed my usual procedure of loading the projects into Visual Studio 2010 and set the target for all projects to use .Net 4.0 framework. So far so good. I tested it on my development machine which is Windows 7 and everything worked as I expected.
So I went ahead with deployment on my production Windows server. I changed the target ASP.Net framrwork of my web site to ASP.Net 4.0 from ASP.Net 2.0. And site loaded and started serving requests. After few hours I got an email from another site owner who is hosting site on my server that all sites have stopped working and we were getting Server Error or Application Error. I have about dozen sites hosted on the server and all but one stopped working. Only one that was working was this site that I just upgraded to use ASP.Net 4.0. Right there I realized the mistake I made. This is the samee mistake that was made when sites were upgraded from ASP.Net 1.1 to ASP.Net 2.0.
Yes, now I had one site in the application pool that was using ASP.Net 4.0 and others were using ASP.Net 2.0. Application pools do not allow mixed version of .Net framework. Here are some things that you will have to take into consideration when migrating your existing ASP.Net sites from 2.0 t0 4.0 or when adding new sites on Windows 2003 server.
I hope this little piece of information helps you get out of problem of upgrading existing site to ASP.Net 4.0 when there are other sites in same pool that use older version of ASP.Net.