Take a look at the following. As part of maintenance on the forum, I sometimes check for topics with no tags, and then add tags. I had 6 topics with no tags, and when I clicked through, to 5 of them, there were no tags, so I tagged them. This one though shows in the list of topics with no tags, but when I click through, there are lots of tags.
Any ideas?
Link to screenshot of the topic with no tags
Link to screenshot of the actual tags for this topic