In February a wedding party reserves eight of your twelve doubles for a Saturday in June. Nobody has named a guest yet — the couple will send a rooming list in May. Your system records the block and shows the rooms as held.
In April a regular calls and asks for that Saturday. The desk checks the board. Four doubles are free, so it sells one. The screen was telling the truth about rooms, and lying about inventory.
The reason is subtle and almost universal. A group block holds a count per room type per night. It owns no room number and no reservation row, deliberately, because a block has to remove rooms from the whole house rather than cap one channel. So a check that asks "is room 7 free?" cannot see it. Only a check that asks "can this room type still be sold?" can.
The bill arrives in June: the couple's rooming list needs eight and you have seven. Somebody is being walked, on a wedding night, at your expense and in front of forty guests.
Why this matters to you: the failure is silent for four months and then surfaces at the single worst moment. A property needs its availability answer to include pooled holds, allotment ceilings and stop-sells — and it needs every path that sells a room to ask the same question, including the public website and the walk-in at the desk. One guard, no exceptions, is the only version of this that works.