Also called: tracking drift, tag rot, silent tracking failure
Drift is silent by construction. Analytics platforms report what they receive; they have no way to report what they should have received. A purchase event that stopped firing produces a clean, plausible chart with a lower line on it.
The three highest-frequency causes are a theme or template deploy that drops the container snippet from one page type, a CSS class rename that invalidates a click trigger, and a consent-banner change that starts denying a signal it previously granted.
Detection needs an external reference point: compare the configured intent (the tracking plan, the container definition) against observed reality (what the live page actually loads and fires). Neither half alone can see the gap.
A broken purchase tag is usually caught at month-end reporting — three to four weeks of ad bidding against an incomplete conversion signal, on top of the reporting gap that cannot be backfilled.
Compare configured tracking against live behaviour on a schedule: crawl the site, record which tags load and which events fire, and diff that against your tracking plan. Alert on the diff, not on traffic volume — volume alerts miss a tag that breaks during a genuine traffic dip.
Continuously for revenue-critical events, and always immediately after a site deploy, a theme change, a plugin update, or a consent-banner change. Those four events cause the large majority of breakage.
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