Not the most difficult, but one of the most satisfying... Early 90's (when C++ was new and broken in a different way for each compiler), I was on a QA team for

Not the most difficult, but one of the most satisfying... Early 90's (when C++ was new and broken in a different way for each compiler), I was on a QA team for a project where developers were learning C++ as they built the next version of the product. There was a buggy math function in the standard lib, and the compiler vendor didn't see it as a high priority. Devs didn't know how to find expressions that were at risk so they could cast them to a type for which the libraries worked reliably. I discovered a useful combination of compiler flags and wrote an awk script to take the compiler output and make a list of source files and lines that produced calls to the broken lib function. The lead developer insisted that I was wasting their time with a bogus list, until I explained how it worked. More challenging: I worked on an industrial machine that had to mix measured amounts of gasses over time. The developer who wrote the mass flow controller (the device that controls gas flows, basically) task just opened the MFC at the start time, then slammed it shut after the correct amount of gas had passed. I coded up a smooth open/close that kept the area under the curve correct. In 6809 assembly language. That was in the early 80s, when a 2MHz 8-bit CPU was some serious horsepower.