Commission, channel management, overbooking and pricing — the operational problems that cost real money.
The goal is not zero commission. It is shifting the guests who would have found you anyway onto a channel that does not charge 18% for the privilege.
Deliberate overbooking is a revenue strategy. Accidental overbooking is a systems failure, and it is the expensive one.
Occupancy and rate can each be pushed at the expense of the other. RevPAR is the number that cannot be gamed.
A blanket two-night minimum is the most common way small hotels lose money while believing they are protecting it.