There's always someone on HackerNews who has done something pertinent to almost any discussion (it's one of the best things about being here, in fact) - are there any COBOL guys or gals who made a fortune fixing Y2K bugs who'd like to share their story?
I never quite understood why the transition from 1999-2000 should be a big deal for a computer system, until I learned about how COBOL works: it stores numbers as its string-representation unless told otherwise, and trying to store 100 in a field of two bytes will happily be stored as "00". Of course we had other bugs, with the same cause, well after the y2k-period.
Not quite literally strings, it typically uses binary-coded-decimal (BCD) format for numbers, but it has the same effect when years are stored as two digits.
I work with a database that has its origins in the COBOL era. All of the date fields are specified in the copybooks as four PIC 99 (i.e. two decimal digits) subfields, CCYYMMDD. This separation of CC and YY surprised me until I realized that it allowed them to add Y2K support by setting the default for CC to '19' and switching it later.
It's not just cobol, there are still plenty of devs out there storing dates as strings. Aside from using more space, most won't notice until the try and filter on a date range.