Exception: Loading this assembly would produce a different grant set from other instances

From http://www.neilpullinger.co.uk/ :
If you get this exception message when trying to browse an ASP.NET site you are developing, first stop the site in IIS, then clear out the ASP.NET Temporary Files folder that relate to your site (default location is under C:WindowsMicrosoft.NETFrameworkv2.0.50727Temporary ASP.NET Files). Then start IIS and try browsing the site.