Overbooking means holding more reservations than rooms available for a given night. In larger hotels it is often a deliberate strategy: based on historical no-show and late-cancellation rates, revenue managers accept a few extra bookings so the hotel does not end up with empty rooms it could have sold.
Unintentional overbooking is a different problem: two channels selling the last room at the same time because availability was not synchronised, or a manual change that never reached the OTAs.
In practice: if you overbook on purpose, base the number on your own data by day of week and segment, and have a walk policy ready (a comparable hotel nearby, transport, a gesture for the guest). If it happens by accident, the fix is a system that blocks double allocation at data level and a live connection between your PMS and every channel.
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