While refactoring, I have to understand what the component does or should do. I read the original code and sometimes the commit history for certain lines. Then I write tests to ensure those assumptions carry to the new code.

More than once, I ended up finding bugs in the old code this way. The ticket said one thing (ticket IDs in commit messages are a godsend), and the code did another thing. There was also a lot of unexpected business logic that was discovered after the first draft of the code, and not documented.

I had the same experience translating text content into calculators. Text can be ambiguous, but code can't. Text had errors that coded logic wouldn't allow.