The Swedish social security number system claims to use a "modulus 10" algorithm to validate numbers. However, the algorithm in use is not what any math savvy person would expect given that description namely "36 modulus 10 is the remainder in division by 10 i.e. 6". But rather "Round to next higher multiple of 10, then subtract" so 36 modulus 10 in the Swedish system is 40-36=4 not 6. So that is actually 10 minus modulo 10. How odd to have an elementary math error in a national standard.