This seems to be an error in the ASP.net runtime.

It can happen with MasterPages or UserControls.  Often it will happen after you update a file and place it on the server.   ASP.net is not able to always compile properly.

There is a hotfix from Microsoft for this.

http://support.microsoft.com/kb/915782

There is a workaround for this issue:

<configuration>

   <system.web>
       <compilation debug="false" batch="false"></compilation>
   </system.web>

</configuration>