Sys.WebForms.PageRequestManagerParserErrorException in Sharepoint

I have just tripped over this (again), the reason;

<add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />

had not been added to the httpModules section of the web.config file.