Index: src/wp-includes/functions.php
===================================================================
--- src/wp-includes/functions.php	(revision 37282)
+++ src/wp-includes/functions.php	(working copy)
@@ -1854,7 +1854,7 @@
  * @return array See above for description.
  */
 function wp_upload_dir( $time = null, $create_dir = true, $refresh_cache = false ) {
-	static $cache = array();
+	static $cache = array(), $tested_paths = array();
 
 	$key = sprintf( '%d-%s', get_current_blog_id(), (string) $time );
 
@@ -1874,13 +1874,10 @@
 
 	if ( $create_dir ) {
 		$path = $uploads['path'];
-		$tested_paths = wp_cache_get( 'upload_dir_tested_paths' );
 
-		if ( ! is_array( $tested_paths ) ) {
-			$tested_paths = array();
-		}
-
-		if ( ! in_array( $path, $tested_paths, true ) ) {
+		if ( array_key_exists( $path, $tested_paths ) ) {
+			$uploads['error'] = $tested_paths[ $path ];
+		} else {
 			if ( ! wp_mkdir_p( $path ) ) {
 				if ( 0 === strpos( $uploads['basedir'], ABSPATH ) ) {
 					$error_path = str_replace( ABSPATH, '', $uploads['basedir'] ) . $uploads['subdir'];
@@ -1889,10 +1886,9 @@
 				}
 
 				$uploads['error'] = sprintf( __( 'Unable to create directory %s. Is its parent directory writable by the server?' ), esc_html( $error_path ) );
-			} else {
-				$tested_paths[] = $path;
-				wp_cache_set( 'upload_dir_tested_paths', $tested_paths );
 			}
+
+			$tested_paths[ $path ] = $uploads['error'];
 		}
 	}