Index: src/wp-admin/includes/class-wp-upgrader.php
===================================================================
--- src/wp-admin/includes/class-wp-upgrader.php	(revision 29684)
+++ src/wp-admin/includes/class-wp-upgrader.php	(working copy)
@@ -1252,32 +1252,36 @@
 
 		if ( empty( $language_updates ) ) {
 			$this->skin->header();
 			$this->skin->before();
 			$this->skin->set_result( true );
 			$this->skin->feedback( 'up_to_date' );
 			$this->skin->after();
 			$this->skin->bulk_footer();
 			$this->skin->footer();
 			return true;
 		}
 
 		if ( 'upgrader_process_complete' == current_filter() )
 			$this->skin->feedback( 'starting_upgrade' );
 
-		// Remove any existing package checks and then set the new one for translations, #WP29230.
+		// Remove any existing upgrade filters from the plugin/theme upgraders #WP29425 & #WP29230
+		remove_all_filters( 'upgrader_pre_install' );
+		remove_all_filters( 'upgrader_clear_destination' );
+		remove_all_filterS( 'upgrader_post_install' );
 		remove_all_filters( 'upgrader_source_selection' );
+
 		add_filter( 'upgrader_source_selection', array( $this, 'check_package' ), 10, 2 );
 
 		$this->skin->header();
 
 		// Connect to the Filesystem first.
 		$res = $this->fs_connect( array( WP_CONTENT_DIR, WP_LANG_DIR ) );
 		if ( ! $res ) {
 			$this->skin->footer();
 			return false;
 		}
 
 		$results = array();
 
 		$this->update_count = count( $language_updates );
 		$this->update_current = 0;