Sometimes a different denomination (or rogue denomination) is mistakenly put within a stack of sorted (single denomination) banknotes. Fortunately banknote counters such as the ZZap NC20 recognise the size of each banknote. Every denomination has a different size (except US dollar) therefore if a different denomination is counted the ZZap NC20 will pause counting and alert you with a visual and audio warning.
High specification banknote counters such as the ZZap NC70 use advanced contact image sensors (CIS) that enable them to scan an image of the banknote. This allows them to sort the banknotes according to denomination, face and orientation. The ZZap NC70 also uses 2 stackers, so it can sort banknotes without stopping.