In the 1990s I did some maintenance work on a process-control system (HP1000 minicomputers, all of 2MB main memory - running about 100 processes - amazing what one could do in the old days).
One of the original developers had passed into management, which was just as well as while a very nice person had limited coding skills. He had done a lot of the Pascal code in the system, and had fallen in love with CASE - it was used everywhere, almost to the exclusion of IF, and the code was peppered with 100 line long (or more!) CASE constructs. In that system the assembly code was generally much better written, commented, and documented than the Pascal code.
One of the tasks in this project was to move to 50MB disk drives, which meant a different driver set & thus different OS (RTE) memory layout, requiring a complete rebuild of the system from source.
Except - one little time-difference calculation function was missing. So we (myself, another young'un, and the boss) looked at the code that was using it, reverse-engineered its functionality, and re-created it (all of 15 lines IIRC).
However... it was our bad luck that the program on which the big rebuild's compilation had failed was a G-code masterpiece, and when the system went live _every other_ program which used TDIFF was getting wrong date calculation results.