customers able to purchase more than what is in stock

[LissyNEWZEALAND]

hi everyone - even though i have some products set at a certain amount in terms of inventory, customers are still able to purchase more than that and add it to their cart. for example, if i have set a certain dining chair to 5 left in stock, i've made sure that "Continue selling when out of stock" is UNCHECKED however still people can add say, 15 to their cart with no error..

please help!