Ask HN: When coding, what are examples of things that you want to remember later

When you are focused and coding, what are some things that crop up in your head which, at that instant, you feel "I want to remember this for later" I don't just mean in terms of todos for later, but also I remember this now, but I won't if I look at it 3 weeks from now. For example, it could be decision that you made while coding, where you took one approach instead of others. Why you took that approach and not others is something in your head at that moment but takes time to remember in the future.