What is your early warning sign that a small problem is about to become a big one?

Every business I have run has had a version of the same warning sign. A customer complaint that repeats instead of a one time thing. A number that drifts slightly off for a few weeks in a row. A task that keeps getting rescheduled instead of finished. None of those look urgent on the day they happen. They only look obvious in hindsight, once they have already turned into something bigger. Curious what your early warning sign looks like, the thing that in hindsight was always the tell, but did not feel like one at the time.