Working with a coworker to understand and then rein in an ancient class hierarchy where much of the naming and documentation are misleading and/or no longer apply, and...
If you have never worked in a large, older codebase that has grown organically, lemme just say that it is an EXPERIENCE.
It's an art as much as a science, and if you don't keep things tidy it's *so* easy to snowplow tech debt.