If you are using Bundles with ASP.NET, your generated JS or CSS reference will look something like this:

<link href="/bundles/css?v=cpIM3_ePEQRK4Cm2oRdV4r0frJ3SdX2kHiKOGjdViSA1" rel="stylesheet">;

However, because all requests go through Sitecore’s HttpRequest pipeline, Sitecore will return a ‘Page not found’ error for that dynamically generated URL.

To avoid this problem, add the root of any dynamically generated content (‘bundles’ in my example) to the IgnoreUrlPrefixes setting in Sitecore’s web.config:

<setting name="IgnoreUrlPrefixes" value="/sitecore/default.aspx|/trace.axd|/webresource.axd|/sitecore/shell/Controls/Rich Text Editor/Telerik.Web.UI.DialogHandler.aspx|/sitecore/shell/applications/content manager/telerik.web.ui.dialoghandler.aspx|/sitecore/shell/Controls/Rich Text Editor/Telerik.Web.UI.SpellCheckHandler.axd|/Telerik.Web.UI.WebResource.axd|/sitecore/admin/upgrade/|/layouts/testing|/bundles">

Sitecore will not attempt to process this URL and your CSS/JS will work.