diff --git a/src/wp-includes/post.php b/src/wp-includes/post.php
index 98aeef597f..ccb11de728 100644
--- a/src/wp-includes/post.php
+++ b/src/wp-includes/post.php
@@ -6126,11 +6126,11 @@ function wp_add_trashed_suffix_to_post_name_for_post( $post ) {
 
 	$post = get_post( $post );
 
-	if ( strpos( $post->post_name, '-%trashed%' ) ) {
+	if ( '__trashed' === substr( $post->post_name, -9 ) ) {
 		return $post->post_name;
 	}
 	add_post_meta( $post->ID, '_wp_desired_post_slug', $post->post_name );
-	$post_name = _truncate_post_slug( $post->post_name, 190 ) . '-%trashed%';
+	$post_name = _truncate_post_slug( $post->post_name, 191 ) . '__trashed';
 	$wpdb->update( $wpdb->posts, array( 'post_name' => $post_name ), array( 'ID' => $post->ID ) );
 	clean_post_cache( $post->ID );
 	return $post_name;
diff --git a/tests/phpunit/tests/post/wpInsertPost.php b/tests/phpunit/tests/post/wpInsertPost.php
index deb95701cc..f0e150a8c8 100644
--- a/tests/phpunit/tests/post/wpInsertPost.php
+++ b/tests/phpunit/tests/post/wpInsertPost.php
@@ -15,7 +15,21 @@ class Tests_WPInsertPost extends WP_UnitTestCase {
 			'post_status' => 'publish'
 		) );
 		wp_trash_post( $trashed_about_page_id );
-		$this->assertEquals( 'about-%trashed%', get_post( $trashed_about_page_id )->post_name );
+		$this->assertEquals( 'about__trashed', get_post( $trashed_about_page_id )->post_name );
+	}
+
+	/**
+	 * @ticket 11863
+	 */
+	public function test_trashed_suffix_should_be_added_to_post_with__trashed_in_slug() {
+		$trashed_about_page_id = self::factory()->post->create( array(
+			'post_type' => 'page',
+			'post_title' => 'About',
+			'post_status' => 'publish',
+			'post_name' => 'foo__trashed__foo',
+		) );
+		wp_trash_post( $trashed_about_page_id );
+		$this->assertEquals( 'foo__trashed__foo__trashed', get_post( $trashed_about_page_id )->post_name );
 	}
 
 	/**
@@ -49,7 +63,7 @@ class Tests_WPInsertPost extends WP_UnitTestCase {
 			'post_status' => 'publish'
 		) );
 
-		$this->assertEquals( 'about-%trashed%', get_post( $trashed_about_page_id )->post_name );
+		$this->assertEquals( 'about__trashed', get_post( $trashed_about_page_id )->post_name );
 		$this->assertEquals( 'about', get_post( $about_page_id )->post_name );
 	}