I recently reviewed a suggested edit and this is what I saw:
As you can see, spells appears twice, once as though it were removed, and once as though it were added. Is this a bug? What does this mean?
Main meta Q on this: Diff shows tag being added and removed in the same revision
The answer to which explains:
The tagging system automatically sorts tags based on how many times they're used. However, this is relatively new, so when a new edit is processed for an old post, the system sees the sort as a delete and re-insert because it is now in a new position.