SELECT topic_id,topic_title,topic_time,topic_views, username, forum_id FROM manual_topics, phpbb_users WHERE (forum_id = 26 OR forum_id = 27) AND user_id = topic_poster ORDER BY topic_time Desc LIMIT 0, 5";
AND precede a OR, con lo que no poner los paréntesis equivaldría a
WHERE forum_id = 26 OR (forum_id = 27 AND user_id = topic_poster)
Espero que ahora si te funcione ;)
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