Maybe I've found a related issue.
In this release-group, you can see liste de courses jpn and liste de courses tags.
Those really look like tags of mine, I highly doubt someone else would use that.
But on this RG, my tag is now only liste de courses jpn (2).
If I use one of these tags, the usage count goes 2.
The old tags are still there, associated to a ghost?, although I guess no one has used them (but we can't see who tagged any more, right?).

Please open a new ticket for the behavior you are seeing, it's not about this ticket.

I thikn it has to do with the « garbage collect » part of this ticket, maybe.

What you're seeing is not a bug at all. Someone else has used those tags - http://musicbrainz.org/user/erpbridge/tags/liste%20de%20courses and http://musicbrainz.org/user/erpbridge/tags/liste%20de%20courses%20jpn

Agreed to fix within 12 months by http://scheduling.ocharles.org.uk and http://chatlogs.musicbrainz.org/musicbrainz-devel/2012/2012-03/2012-03-26.html#T19-36-54-3898

In review at http://codereview.musicbrainz.org/r/2245/

This is in the beta branch, but these changes are only on the database, so they can't be tested until we deploy in 2012-09-17.