Framework .NET 3.5 SP1 Little Secret

If you get this error:

Could not load type 'System.Web.UI.ScriptReferenceBase' from assembly 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.

Try installing Framework .NET 3.5 SP1. It usually bring problems when passing to production since VS 2008 brings this version and the version on the production server sometimes is not fully up to date.