How it happens
The condition guarding the event tightens or loosens. `if (user.isActive && user.daysSinceSignup > 7)` collapses to `if (user.isActive)`. The event still fires, just for a wider population than before.
What breaks downstream
Cohort definitions silently shift. A retention metric that used to measure week-one-active users now measures all-active users. The number changes; nobody knows why. Or worse, nobody notices.