Index: wp-includes/widgets.php
===================================================================
--- wp-includes/widgets.php	(revision 11157)
+++ wp-includes/widgets.php	(working copy)
@@ -1034,6 +1034,7 @@
 	}
 
 	if ( $single ) {
+		$found = false;
 		$settings = array( 2 => $settings );
 
 		// If loading from the front page, update sidebar in memory but don't save to options
@@ -1046,16 +1047,24 @@
 		}
 		
 		foreach ( (array) $sidebars_widgets as $index => $sidebar ) {
+			if ( $index == 'wp_inactive_widgets' )
+				continue;
+
 			if ( is_array($sidebar) ) {
 				foreach ( $sidebar as $i => $name ) {
 					if ( $base_name == $name ) {
 						$sidebars_widgets[$index][$i] = "$name-2";
+						$found = true;
 						break 2;
 					}
 				}
 			}
 		}
-
+		
+		// don't register unfound, single widgets
+		if ( !$found )
+			$settings = array();
+		
 		if ( is_admin() )
 			update_option('sidebars_widgets', $sidebars_widgets);
 	}