Steps to reproduce the issue

$db = JFactory::getDbo();

	// Create a new query object.
	$query = $db->getQuery(true);
		
	$fieldlist = $db->qn(array('category.id','category.title'), array('id', 'title')); // add the field names to an array
	$fieldlist[0] = 'distinct ' . $fieldlist[0]; 
		
	$query->select($fieldlist);
	$query->from($db->quoteName('#__categories', 'category'));
	$query->join('INNER', $db->quoteName('#__content', 'article') . ' ON (' . $db->quoteName('category.id') . ' = ' . $db->quoteName('article.catid') . ')');
	$query->where("article.state = 1");
	$query->order("title");

	// Reset the query using our newly populated query object.
	$db->setQuery($query);
		
	// Load the results as a list of stdClass objects
	$results1 = $db->loadObjectList();

	$q = $db->getQuery(true);

	$otherFieldlist = $db->qn(array('category.docman_category_id','category.title'), array('id', 'title')); // add the field names to an array
	$otherFieldlist[0] = 'distinct 0-' . $otherFieldlist[0]; 

	$q->select($otherFieldlist);
	$q->from($db->quoteName('#__docman_categories', 'category'));
	$q->join('INNER', $db->quoteName('#__docman_documents', 'document') . ' ON (' . $db->quoteName('category.docman_category_id') . ' = ' . $db->quoteName('document.docman_category_id') . ')');
	$q->where("document.enabled = 1");
	$q->union($query);
	$q->order("title");

	$db->setQuery($q); die($q);

Generate an exception :

Incorrect usage of UNION and ORDER BY SQL=SELECT distinct 0-category.docman_category_id AS id,category.title AS title FROM #__docman_categories AS category INNER JOIN #__docman_documents AS document ON (category.docman_category_id = document.docman_category_id) WHERE document.enabled = 1 ORDER BY title UNION ( SELECT distinct category.id AS id,category.title AS title FROM #__categories AS category INNER JOIN #__content AS article ON (category.id = article.catid) WHERE article.state = 1 ORDER BY title)

Because of parenthesis needed to encapsulate the first query !

Expected result

(SELECT distinct 0-category.docman_category_idASid,category.titleAStitleFROM#__docman_categoriesAScategoryINNER JOIN#__docman_documentsASdocumentON (category.docman_category_id=document.docman_category_id) WHERE document.enabled = 1 ORDER BY title) UNION ( SELECT distinctcategory.idASid,category.titleAStitleFROM#__categoriesAScategoryINNER JOIN#__contentASarticleON (category.id=article.catid) WHERE article.state = 1 ORDER BY title)

Actual result

SELECT distinct 0-category.docman_category_idASid,category.titleAStitleFROM#__docman_categoriesAScategoryINNER JOIN#__docman_documentsASdocumentON (category.docman_category_id=document.docman_category_id) WHERE document.enabled = 1 ORDER BY title UNION ( SELECT distinctcategory.idASid,category.titleAStitleFROM#__categoriesAScategoryINNER JOIN#__contentASarticleON (category.id=article.catid) WHERE article.state = 1 ORDER BY title)

System information (as much as possible)

Joomla! 3.6.5
Base de données : MySQL 5.7.19

Additional comments