Sometimes I just click the Play(Start) button in Visual Studio.NET when I want to lauch and debug my application. I'm using NHibernate for the first time and sometimes I get the following error.

CS0006: Metadata file 'c:\windows\microsoft.net\framework\v1.1.4322\temporary asp.net files\requirements\e8bd7fc8\c8bd704c\assembly\dl2\c7c4ad9d\00cafe4f_2893c501\castle.dynamicproxy.dll' could not be found

When I do a "rebuild solution" and click start the message does not appear and the web app starts as expected.

Has anyone else seen this or may know why I'm receiving this error ?

This is by design according to Microsoft. The reason is that you cannot use strong signed assemblies in the bin directory. Just install castle.dynamicproxy.dll in the GAC to get rid of this error.