There have been quite a few. The mods are usually pretty good at weeding them out. There was one post that inspired another thread praising the post. 11 people, on the new thread, said how good the post was...how it 'spoke' to them. The post being praised was anti-Semitic and deleted. The thread praising the post was locked with a message from the mods stating that they were locking the thread because, after discussion, they (the mods) deemed the post being praised as anti-Semitic.
On edit: Here is the thread...
http://www.democraticunderground.com/discuss/duboard.php?az=show_topic&forum=104&topic_id=3786654#3793912