Probably because the second fetch of the page to get the combined script file ends up including Trace output. Probably workarounds: set CombineScriptsHandlerUrl OR CombineScripts=false.
Comments: I think I've finally figured this out. My project is a .Net 4.0 WebForms application that uses Routing. It looks like the Routing module was picking up the request before the handler got involved. I have fixed it by adding a specific ignore route for *.ashx files: RouteTable.Routes.Ignore("{resource}.ashx/{*pathInfo}");
Comments: I think I've finally figured this out. My project is a .Net 4.0 WebForms application that uses Routing. It looks like the Routing module was picking up the request before the handler got involved. I have fixed it by adding a specific ignore route for *.ashx files: RouteTable.Routes.Ignore("{resource}.ashx/{*pathInfo}");