If an air traffic controller is found asleep on the job, an immediate reaction would be to fire the individual, and go no further: a problem has been identified and now it has been removed.

But let’s say, hypothetically, that the air traffic controllers was making up for a sick colleague and was doing a double shift; and wanted to nap 15 hours into the job.

Seen in this way, the initial observation is not the core cause or problem, but instead a symptom of another (set of) problems – lack of sufficient staff members, poor scheduling, or scheduled breaks during shifts.

This particular example is described on Caseinterview.

It seems like a useful skill to be able to analyse a situation from different perspectives and distinguish what is a cause from what is a symptom, and separate the two.