Busiest when nobody comes
The puzzle
People say this restaurant is doing best when no customers come.
When one does come, the owner looks troubled.
See the answer (spoiler)
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Answer
It takes bookings only, and every table is reserved.
"No customers coming" means nobody walks in off the street — which is what a full book looks like from the pavement.
A walk-in has to be turned away, so the owner looks troubled.
