Definition
Instrumentation drift is when the analytics calls in your repo change in ways nobody intended. A capture call disappears in a refactor. An event name gets tidied. A property changes type. The product still runs and the dashboards still draw; the data quietly degrades.
Five shapes of drift
Removed events, renamed events, moved events, altered payloads, and conditional firing changes. Each is technically a code change like any other, and each silently corrupts a downstream analytics surface.