The A-not-B error is an incomplete or absent schema of object permanence, normally observed during the sensorimotor stage of Jean Piaget's theory of cognitive development.
A typical A-not-B task goes like this: An experimenter hides an attractive toy under box "A" within the baby's reach.
This activity is usually repeated several times (always with the researcher hiding the toy under box "A"), which means the baby has the ability to pass the object permanence test.
Children of 12 months or older (in the preoperational stage of Piaget's theory of cognitive development) typically do not make this error.
They found that various components of the activity (strength of memory trace, salience of targets, waiting time, stance) combine in the "B"-trial (where the object is hidden in the "B" location rather than "A") so the child either correctly or incorrectly searches for the toy.