This happened to me a couple of times, and I learned the hard way.
It’s not just ideas about writing. Ideas about absolutely anything and everything will and does vanish from my mind if I don’t note it down right away.
Sometimes I wonder how different my life would be if I had written down all the ideas I had since childhood and took action on at least 1% of them. Ah!
Lately, whatever comes to mind ends up in my Medium drafts. You don’t wanna see my drafts page. It’s everything my bedroom isn’t. I have way too many headlines and abstracts sitting there waiting for me to take action.
The sheer size of the drafts list is another source of frustration. Still an easier one to deal with than that of a forgotten idea.