Usually, when I go back to my old code, I would find that there is room for improvement, so I would try to improve it. But most of the time, it dawns on me that my past self was already knowing that there was room for improvement but it's simply illogical to do because there would be an edge case bug. There is always reasoning that makes sense. So I would waste my time discovering that edge-case bug that my past self already knew. The edge-case bug simply eliminates the reasoning of the improvement completely, it's like you know the earth is round and not flat anymore.
When you thought you know better, so you try to come up with something smart, but it turns out that someone else already knows it and you just discovering the same thing later. What is this process of discovering called?
I want to use the word as a topic for the comment. E.g. "X: Don't try to change this code because of A, B, C, etc" I want to know what is X. It will also help me search for related topics later.
The exact word for discovering something again you'd since forgotten is "rediscover". So, if you want a noun, "rediscovery" would be appropriate. For your intended use, something more like "reminder" or possibly even "warning" or "todo" could work as well. Really, you want to prevent "rediscovery" by leaving this comment.