If you connect an event handler to a TAdvColumnGrid's OnEditCellDone event, a combo box will generate an event where the Row and Column are exchanged.
TAdvStringGrid.DoComboCloseUp calls OnComboCloseUp(Self, ARow, ACol), which is automatically connected to the grid's OnEditCellDone event. However OnEditCellDone expects the parameters to be in the following format.
TEditCellDoneEvent = procedure(Sender: TObject; ACol, ARow: Integer) of object;
To work around this issue, I've hacked the DoComboCloseUp method and reversed the order of the Row/Column parameters.
This was a noob error on my part. I hooked the OnComboCloseUp event to my OnEditCellDone handler and they're not compatible.