Index: wp-admin/post.php
===================================================================
--- wp-admin/post.php	(revision 24157)
+++ wp-admin/post.php	(working copy)
@@ -225,6 +225,9 @@
 
 	$post = get_post($post_id);
 
+	if ( !$post )
+		wp_die( __('The item you are trying to move to the Trash no longer exists.') );
+
 	if ( !current_user_can($post_type_object->cap->delete_post, $post_id) )
 		wp_die( __('You are not allowed to move this item to the Trash.') );
 
@@ -243,6 +246,9 @@
 case 'untrash':
 	check_admin_referer('untrash-post_' . $post_id);
 
+	if ( !$post )
+		wp_die( __('The item you are trying to restore from the Trash no longer exists.') );
+
 	if ( !current_user_can($post_type_object->cap->delete_post, $post_id) )
 		wp_die( __('You are not allowed to move this item out of the Trash.') );
 
@@ -256,6 +262,9 @@
 case 'delete':
 	check_admin_referer('delete-post_' . $post_id);
 
+	if ( !$post )
+		wp_die( __('This item has already been deleted.') );
+
 	if ( !current_user_can($post_type_object->cap->delete_post, $post_id) )
 		wp_die( __('You are not allowed to delete this item.') );