But in most real codebases separate units don't interact cleanly. "you look at the connections between the modules and see whether any data can possibly flow fr

But in most real codebases separate units don't interact cleanly. "you look at the connections between the modules and see whether any data can possibly flow from one to the other." is something that would be very hard to do in codebases that aren't perfect. Probably more of a waste of the mental cycles than just putting it right if it is a quick fix. With version control you can even avoid the problems of making the original problem worse with your fix.