Oh the joys of data formats
Somewhat along the lines of this story...
Every danish citizen has a unique identity-number issued at birth. System was designed in the 1960's, so obviously had to carefully consider how much data to store - meaning they ended up with a number including the date of birth in the DDMMYY format: DDMMYY-NNNN, the last 4 digits being a sequence number.
Except it wasn't quite a sequence number, because some bright fellow decided that it would be nice to distinguish between men and women, so the last digit is odd for men and even for women. (You can guess how the transgenders feel about that). Another bright fellow discovered that in 1960 they actually had grandparents born in the 1800's, so the first digit of the sequence number was used to encode the century: 0-4 if you were born in the 1900's, and 5-9 for the old people from the 1800's. Guess how that worked once year 2000 turned up, and we still had some people alive from the 1800's.
As the final twist, the sequence number also acted as a checksum of the entire identity number, with each digit multiplied by a specific factor, added together, and the sum had to be divisible by 11. Bizarre, and with the additional "feature" that you can only have about 250 people born on any one day. This wasn't really a problem until people started arriving from countries where you really don't care much about when you were born, so a third bright fellow decided that if the date of birth was unknown, assume Jan 1st of a year that seems plausible. Guess what happened when a surge of asylum seekers arrived one day...
So the checksumming was abandoned. But the identity number is used by every single public and private sector business, so quite a bit of scrambling when they had to remove that check from the customer entry forms.
Public sector IT disasters - you cannot make them up, they are for real.