ok, a bit academic but a neighbour's child asked what would happen if you wrote out and paid in a cheque to yourself (same account).
My immediate reaction was that the bank would find a way of holding the money for 4 days and the account would be down by the amount of the cheque.
Another neighbour (bank worker) said that the effect would be neutral as the bank balance would go up on paying the cheque in and then go down when it was cleared.
So what's the real answer?