I'm thinking there's a bug looming.
I just edited an older post that had tags.
I didn't touch the tags in any way, but when I went to update the post I got slammed with this error. BTW the post didn't update.

Additional information about this error:

MySQL error!

Duplicate entry 'firefox' for key 'tag_name'(Errno=1582)

Your query: insert new tags

INSERT INTO evo_items__tag( tag_name )

VALUES ( 'firefox' ),( 'gmail' ),( 'google' ),( 'mac' )

The only way to edit a post is to delete it's related tags.

EDIT: Related post in the [URL=http://forums.b2evolution.net/viewtopic.php?t=13482]bugs forum[/URL]

hmm... the old bug relives again...
in anycase, haven't been touching the new tag feature since this bug first appeared.

And I'm still thinking how best to combine Tagging and Categories.

Anyway... not much I can help here... sorry for OT.