[entwisi]

I can guess at Card numbers even down to Bank level. Basically teh first 6 digits of a card number tell me exactly what bank and type of account it is. (Its called a Bank Identification Number or BIN). The last digits of the card are coded versions of your expiry date.

So simple 'guessing' mechanism only has ~6 or 7 digits to guess for a valid card number.



Another option is simply when you use the card in a shop. Its not hard to memorise all the details of a card including the important 3 digits on the back.
You will often find that people hold these details for a while before using them so as to 'detach' the association from them