To avoid confusion over debits and credits, avoid thinking of them in the way that they are used in everyday language, which often refers to a credit as increasing an account and a debit as decreasing an account. For example, if our bank credits our checking account, money is added to it and the balance increases.
One way to reduce the frequency of this occurrence is to maintain an air pocket at the top of the water heater tank. This air pocket will form in the tank by design - however, it will be reduced over time by the everyday use of your water heater. To replenish this air pocket: Turn off the water heater.
A bad gas valve will not allow any gas to go to the burner or pilot light. See the article on 'Hot Water Heater Gas Valves' for the issues surrounding this device. A clogged or partially clogged vent can cause this problem. See Venting a Gas Water Heater for information on keeping your vent clear.