Last night as I was falling asleep I had an idea for a blog. It was so clear that I had it half-written in my head. I should get up, start up my computer, and start writing. Naw! I would rather go to sleep. I can do it in the morning.
The sun came up and the idea had gone completely away. Just like Adelaide Anne Porter's "The Lost Chord", my lost word "trembled away into silence… I have sought, but I seek vainly". See "The Lost Chord" in Wikipedia.