@cstross As an ex-COBOL programmer who was knee-deep in the Y2K mess, one of the issues is that many of those old COBOL programs were compiled, but the source code was lost.
In some cases, corporations were worried COBOL vendors would disappear, so they created incompatible, in-house COBOL compilers, making the mess even worse!
Naturally, there were no tests to verify that the software functioned correctly ...