The hall is booked twice for the same afternoon
Assemblies, mock exams, rehearsals and visiting speakers all want the same hall, and a diary in the office only finds the clash when two groups arrive at the door.
In ManageCal the hall is a sub-calendar of its own with overlaps turned off. A second booking that collides is refused when someone tries to save it, before anyone has told a class where to go. Rooms that are happy to be shared, like the library, simply leave the setting off.
The rehearsal is refused on save. The hall does not allow overlapping events.