Regression testing with the Command design pattern
Computational mechanics practitioners are not typically known for practising well-established software engineering techniques. That makes code developed by various research groups next to impossible to understand and modify. I still see requests on iMechanica on how to implement Drucker-Prager models when every freshman student probably has their own implementation. It is likely that the person who developed a particular code cannot themselves understand the code a few years after the relevant paper has been published.