Another scenario which can cause overbooking is moving reservations between categories.
The reason why this can be happen is:
If you have unassigned reservations that means there are available spaces shown in your calendar. As such you are able to drag and drop the reservation from one room type to another. If the category was already full this will cause an overbooking.
Please, make a note that the system doesn’t inform you that the overbooking can happen, it only offers you to recalculate the total price or cancel this action.
In order to avoid such type of overbookings, before dragging some reservation, please make sure that for those dates there are no unassigned reservations.
In order to research why this overbooking was allowed, you can use the activity log. If you are still unable to identify or correct the cause of the overbooking, please reply with all the details like: the booking ID, the dates, the rooms, channel affected, confirm which information is incorrect and which you expect to see and we'll be happy to research this for you.