The A-Not-B error, also known as a perseverative error, is an error in the mental perception of objects seen in infants before the age of one year. By the time an infant is around 8 months old they are able to realize that objects that are hidden still exist and have not disappeared. This illustrates a child's ability to mentally represent objects in their mind.
This ability is called object permanence. This ability is fragile in infants and can be manipulated easily. Infants still under the age of one will commit the A-Not-B error. To test this reaction an object is hidden in the same location (location A) multiple times and the infant will find it each time, but once the infant sees the object hidden in a new location (location B) it will typically continue to look for the object in location A. Normally, the infant will begin to correctly search in location B by the time they reach the age of one.