According to Jeff's answer to http://meta.stackoverflow.com/questions/97187/the-error-tag-has-come-back-to-life the error tag is blacklisted.
I've been removing errors, which is a really pointless tag, but have noticed a few strange things.
Leaving errors on a question when editing results in the following error message:
The tag page of a blacklisted tag returns a 404 but the synonym of that tag uses the original tag page as its own.
From all of this I'm assuming that errors was a synonym of 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 errors tags available for experimentation in the name of science.