Credits and debit cards issued in Finland have two checksums instead of just one. The outer checksum is the standard luhn-mod-10 as usual, but the inner checksum is another method (variation of IBM Check with 7 1 3 multipliers). So why is this interesting?
The two checksums make it far easier to resolve partial card numbers. For example if you take a picture of your card by hiding 3 digits with you finger, such masked card number can be resolved to about 12 possibilities.
TL;DR: Never post pictures of your Finnish debit/credit card, even when hiding some of the digits.
I have no knowledge if cards elsewhere might have similar double-checksum feature.
The two checksums make it far easier to resolve partial card numbers. For example if you take a picture of your card by hiding 3 digits with you finger, such masked card number can be resolved to about 12 possibilities.
TL;DR: Never post pictures of your Finnish debit/credit card, even when hiding some of the digits.
I have no knowledge if cards elsewhere might have similar double-checksum feature.