diff --git tests/phpunit/includes/testcase.php tests/phpunit/includes/testcase.php
index 1e8ea11..7abceae 100644
--- tests/phpunit/includes/testcase.php
+++ tests/phpunit/includes/testcase.php
@@ -37,6 +37,7 @@ class WP_UnitTestCase extends PHPUnit_Framework_TestCase {
 		ini_set('display_errors', 1 );
 		$this->factory = new WP_UnitTest_Factory;
 		$this->clean_up_global_scope();
+		$this->reset_taxonomies();
 		$this->start_transaction();
 		$this->expectDeprecated();
 		add_filter( 'wp_die_handler', array( $this, 'get_wp_die_handler' ) );
@@ -67,6 +68,20 @@ class WP_UnitTestCase extends PHPUnit_Framework_TestCase {
 	}
 
 	/**
+	 * Unregister existing taxonomies and register defaults.
+	 *
+	 * Run before each test in order to clean up the global scope, in case
+	 * a test forgets to unregister a taxonomy on its own, or fails before
+	 * it has a chance to do so.
+	 */
+	protected function reset_taxonomies() {
+		foreach ( get_taxonomies() as $tax ) {
+			_unregister_taxonomy( $tax );
+		}
+		create_initial_taxonomies();
+	}
+
+	/**
 	 * Saves the action and filter-related globals so they can be restored later.
 	 *
 	 * Stores $merged_filters, $wp_actions, $wp_current_filter, and $wp_filter