How it happens
A property gets renamed (`plan` to `plan_tier`), dropped, or silently changes type (string to number). Most analytics SDKs accept whatever payload you hand them, so the change ships without errors.
What breaks downstream
Filters by the old property name return zero. Dashboards that grouped by the property now show one big bucket called `null`. Type changes break SQL casts in downstream warehouse models. Through all of it, the event count reads as normal.