This was the case for us when we tried to do Response.Redirect() in Composer function. I would try first to search for any redirect methods in codebase. If found any - try to end output when redirecting (second parameter of the Redirect method). But be careful about ending output - as this will throw ThreadAbort exception and may influence on performance of app.
I am getting the following error quite frequently, however not able to get the root cause of this:
2013-06-18 09:56:46,272 ERROR  EPiServer.Global.Global_Error - 1.2.5 Unhandled exception in ASP.NET
System.Web.HttpException (0x80004005): Server cannot append header after HTTP headers have been sent.
at System.Web.HttpResponse.set_RedirectLocation(String value)
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
Environment: CMS6 R2 with 2 frontend and one backend
However not able to replicate this in our local or staging environments. What can be a possible cause?