Hidden Tag Drift
Ship the tag data and the UI surface together, or don't ship the data. The middle state — data exists, UI doesn't — is the worst of both worlds.
It's tempting to ship a tagging system in the data model and surface it later. This always ages badly. Without UI consuming the tags, contributors invent new ones, drop the controlled vocabulary, and within six months you have three tags for the same concept and no filter to use them on.
The discipline is: ship the data and the surface together, or don't ship the data. Either tags appear as a filter, a related-content rail, or a faceted nav from day one, or they live in a file that's locked and never expanded. The middle state — data exists, UI doesn't — is the worst of both worlds.