It is very strange. After the URL Routing module had been added into the non-MVC Web application, the application kept trying to load Login.aspx into the pages that don't need authentication and cause other syntax errors.
Uncaught SyntaxError: Unexpected token < Register.aspx:63Uncaught Error: ASP.NET Ajax client-side framework failed to load. Login.aspx:3Uncaught SyntaxError: Unexpected token < jsdebug:1Uncaught ReferenceError: Type is not defined
Uncaught SyntaxError: Unexpected token < Login.aspx:187Uncaught ReferenceError: WebForm_AutoFocus is not defined
After a few hours searching in code, I realized that the problem was caused by WebResource.axd. All the scrips for WebForm validation, focus, post back and etc were replaced by Login.aspx. WebResource.axd were instead trying to load Login.aspx.
Another interesting is that I duplicated some of user controls and pages to another temporary project for investigation but I am unable to re-produce the same behavior. With the same routing table, the same Web.config and a scale-down Global.ascx, Login.aspx won't be loaded when it is not being asked for. All JavaScripts are loaded as expected by WebResource.axd without problems. I still wonder what settings or events in the original application cause this problem.
Since this problem is caused by routing, one way to fix is to bypass the route. Adding RouteTable.Routes.RouteExistingFiles = true; won't fix the problem. Instead, a specific rule for ignoring a route is needed.
// For .NET 4.0
//RouteTable.Routes.Ignore("{resource}.axd/{*pathInfo}");
// For .NET 3.5 SP1
RouteTable.Routes.Add(new Route("{resource}.axd/{*pathInfo}", new StopRoutingHandler()));