Index: wp-includes/pluggable.php
===================================================================
--- wp-includes/pluggable.php	(revision 18589)
+++ wp-includes/pluggable.php	(working copy)
@@ -844,6 +844,14 @@
 	$adminurl = strtolower(admin_url());
 	$referer = strtolower(wp_get_referer());
 	$result = isset($_REQUEST[$query_arg]) ? wp_verify_nonce($_REQUEST[$query_arg], $action) : false;
+
+	// Detect errors caused by truncated posts
+	// If the file was uploaded, but exceeds the file upload limit, then it will be handled elsewhere
+	// but if the post was too big, then it won't be decoded properly and the nonce won't come through
+	// so it will be caught here
+	if ( 'POST' == $_SERVER['REQUEST_METHOD'] && empty($_POST) && $_SERVER['CONTENT_LENGTH'] > 0 )
+		wp_die(__( 'The request exceeds the <code>post_max_size</code> directive in <code>php.ini</code>.' ));
+
 	if ( !$result && !(-1 == $action && strpos($referer, $adminurl) === 0) ) {
 		wp_nonce_ays($action);
 		die();