When modifying a system without adequate tests, I found it helps to include debugger breakpoints in my manual testing checklist:

For each change I make, I add a breakpoint after the modified line, and won’t remove it until the interpreter has passed through it exercising the logic to my satisfaction. Before committing the changes I can pull up the list of breakpoints to make sure I’ve hit them all.

This is probably most valuable when I’ve made a lot of changes at once, or if temporary delusion is making me think I don’t need to test everything.

Caveat 1: I suppose, if there’s any parallel operations, you should also test everything with the breakpoints gone, lest the code relies on losing a race condition.

Caveat 2: In no way is this a substitute for automated tests!