According to Jeff's answer to http://meta.stackexchange.com/questions/97187/the-error-tag-has-come-back-to-life the [tag:error] tag is blacklisted.

I've been removing [tag:errors], which is a really pointless tag, but have noticed a few strange things. 

1. [tag:errors] exists despite [tag:error] being blacklisted.
2. Leaving [tag:errors] on a question when editing results in the following error message: 

    >![enter image description here][1]

3. The [tag page of a blacklisted tag returns a 404][2] but the synonym of that tag uses the original tag page as its own.

    >![enter image description here][3]

From all of this I'm assuming that [tag:errors] was a synonym of [tag:error] when it was blacklisted. I think that when a tag gets blacklisted it's synonyms should be burninated or unsynonymised.

I've left the last few [tag:errors] tags available for experimentation in the name of science.

  [1]: https://i.sstatic.net/sBSxz.png
  [2]: http://stackoverflow.com/tags/error/info
  [3]: https://i.sstatic.net/QRadM.png