Make sure prefilters have been called and transport has been selected before sending any global event (in case the "global" option has been changed).

src/ajax.js

@@ -532,14 +532,14 @@ jQuery.extend({
 			s.data = jQuery.param( s.data , s.traditional );
 		}
 
+		// Get transport
+		transport = jQuery.ajax.prefilter( s ).transport( s );
+
 		// Watch for a new set of requests
 		if ( s.global && jQuery.active++ === 0 ) {
 			jQuery.event.trigger( "ajaxStart" );
 		}
 
-		// Get transport
-		transport = jQuery.ajax.prefilter( s ).transport( s );
-
 		// If no transport, we auto-abort
 		if ( ! transport ) {