Stack Exchange sites have an automatic tag clean up system that removes tags with only one use after 6 months. However, it seems like this is detrimental to SciFi Stack Exchange, which has a long tail of questions on less popular / relatively unknown books or series (and surprisingly, some fairly well known ones too).
The result of this tag clean up system is questions being tagged with untagged because they no longer carry any tags. These can be easily cleaned up; what is more worrying is questions which once carried two tags now only carry one generic tag, making it difficult to discover questions from infrequently asked series.
In addition, there are tags which will probably see very few questions. For instance, this question is the only question on this site about the now cancelled TV series FlashForward, so it's unlikely any more flashforward question will be asked. These tags will have to be constantly recreated, since they are legitimate tags, and these questions do deserve the series tags.
So, my proposal is that this system be turned off for SciFi Stack Exchange.
PS. There seems to be an additional problem where the last editor of the question would appear to have introduced the untagged tag into questions, even though it is in fact the system that's doing the job. For example, in the revision history for this question, it might appear that Tony Meyer introduced the untagged tag, but looking at the Tweet you can see that it actually the now-deleted juliet-marillier that was added. This is probably a bug.