Edited Issue: Server cannot append header after HTTP headers have been sent. [556]

Hi All,<br /><br />I have recently started using Web API (version 4.0.20710.0) and have created an ActionFilterAttribute that sets some caching headers in the OnActionExecuted method.<br /><br />I also have a IHttpModule that sets some headers in the PostRequestHandlerExecute event using request.AppendHeader when this happens I get the following error<br /><br />Exception: System.Web.HttpException<br />Message: Server cannot append header after HTTP headers have been sent.<br />Source: System.Web<br /> at System.Web.HttpResponse.AppendHeader(String name, String value)<br /><br />This does not seem to be the case in MVC 3 (haven't tested this in MVC 4). Not sure if I am missing something but looking at the ASP.NET pipeline this should not be the case http://msdn.microsoft.com/en-us/library/ms178473(v=vs.100).aspx<br /><br />Any help would be greatly appreciated.