Index: wp-content/themes/twentythirteen/style.css
===================================================================
--- wp-content/themes/twentythirteen/style.css	(revision 23452)
+++ wp-content/themes/twentythirteen/style.css	(working copy)
@@ -209,7 +209,6 @@
 pre,
 samp {
 	font-family: monospace, serif;
-	_font-family: 'courier new', monospace;
 	font-size: 14px;
 	-webkit-hyphens: none;
 	-moz-hyphens:    none;