If you have an artist with multiple tags, enough that "more..." displays, and you have a tag there of your own which is displaying in the limited # in the sidebar, and you remove that tag, "undefined" displays, instead of "more...".

In this case, I was removing the one tag I had on http://musicbrainz.org/artist/338f9d5d-9327-4f01-bb99-0cdf22c48565