So, your point is that programmers identify the unexpected edge cases through the act of taking their time writing the code by hand. From my experience, it takes a proficient developer to actually plan their code around future issues from separate processes.

I think that it's mistaken to think that reasoning while writing the code is at all a good way to truly understand what your code is doing. (Without implying that you shouldn't write it by hand or reason about it.) You need to debug and test it thoroughly either way, and basically be as sceptical of your own output as you'd be of any other person's output.

Thinking that writing the code makes you understand it better can cause more issues than thinking that even if you write the code, you don't really know what it's doing. You are merely typing out the code based on what you think it should be doing, and reasoning against that hypothesis. Of course, you can be better or worse at constructing the correct mental model from the get go, and keep updating it in the right direction while writing the code. But it's a slippery slope, because it can also go the other way around.

A lot of bugs that take unreasonably long for junior-mid level engineers to find, seem to happen because: They trust their own mental model of the code too much without verifying it thoroughly, create a hypothesis for the bug in their own head without verifying it thoroughly, then get lost trying to reason about a made up version of whatever is causing the bug only to come to the conclusion that their original hypothesis was completely wrong.