[analog31]

Yes, multiple times. And this can happen at a well managed company. To some extent it's my niche. I'm not a coder (I program, but don't touch our product code base), but an industrial scientist, and these projects often involve multiple disciplines such as mechanical and electrical as well as code. My process is to start by just trying to figure out the theory of operation, and identify what is usually a single critical path of data through the code.

Right now I'm looking at a relatively small code base that was written by an engineer in the embedded systems team. The original authors are retired. It's generously documented, and a pleasure to read. This makes my life easier, but also less of a hassle for the un-retired engineers on the receiving end of my requests for help. I don't know about commercial coding, but adding comments and documentation can help a non-specialist work their way through code if they have some decent programming background.

The full re-write can happen if the departed developer is an entire departed business, e.g., an acquired company comes with code that is obfuscated to the point of being Goedel-undecidable ;-)

Science isn't flawless. There's the so called reproducibility crisis. I think what helps science is that nothing of lasting value depends on a single critical path from data to results, but is reinforced by looking at things from multiple angles. For instance we'd still trust general relativity had the Eddington experiment never happened. And scientists such as myself are always looking for ways to make our own work more open and reproducible. In my work, Jupyter Notebook has been a game changer.