Server Error in '/' Application. Server cannot append header after HTTP headers have been sent.

Description: An unhandled exception occurred during the execution of the current web request.

Please review the stack trace for more information about the error
and where it originated in the code.

Exception Details:

System.Web.HttpException: Server cannot append header after HTTP headers have been sent.

Source Error:  An unhandled exception was generated during the execution of the current web request.
Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[HttpException (0x80004005): Server cannot append header after HTTP headers have been sent.]
System.Web.HttpHeaderCollection.SetHeader(String name, String value, Boolean replace) +11971605
Sitecore.Web.XFrameOptionsHeaderModule.EndRequestHandler(Object sender, EventArgs e) +318
System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +142
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +92