The payroll check printer for my employer was once a couple thousand lines that generated raw PCL to be sent to a LaserJet that used magnetic toner to produce checks that had a working MICR number. It was rendered into spaghetti by multiple GOTOs that jumped to helpful labels like "666", and calls into other helper programs to generate more PCL that did things like change fonts and draw graphics. Of course none of it was commented, so you had to have a copy of the PCL spec on hand to know what any of it did. It was the product of a retired cowboy that had also written the rest of our custom payroll system over a number of years.

I attacked it by printing out and taping together each program into "scrolls" and tracing control flow with highlighters and sharpies. Had them all taped up on my office wall so I could refactor the whole thing from scratch, coworkers found that entertaining. Got a much more readable replacement working nicely. Then a couple years later HR bought a new system and we stopped printing our own checks. I was not sorry to see the whole thing go.