Why can stock go negative?

This is by design: checkout is never blocked by insufficient stock. The first rule of a live register is that the customer never waits — a wrong number on paper can be fixed later, but an interrupted sale is lost for good.

Negative stock usually means a restock wasn't recorded, or a pre-ordered item hasn't arrived yet (pre-orders deduct stock when placed). Record restocks by receiving a purchase order and the number recovers on its own; fix stocktake differences with a manual adjustment, which is kept in the movement ledger.