"Rats of Limbo" by Fritz Leiber (from Fantastic Science Fiction Stories, August 1960; never reprinted to my knowledge)
The blurb for this story read, "Every writer must have his fun. One has his fun -- and gives you some, too -- in this little tale, fable, sketch, scrap (crumb?)." I read this 3-page crumb a bit more than sixty years ago and it has stayed with me since then -- that was when I realized that Leiber was a great writer, and my opinion has never changed.
I won't describe it because I there is no way I can do it justice. Suffice it to say that it is a chilling, surrealistic tale of the consequences of having both a bad memory and a most powerful imagination.
Below is a link from Internet Archive for the entire August 1960 issue of Fantastic*. The story begins on page 33. See if your opinion matches mine.
[*Also in this issue are stories by Robert Bloch, Eric Frank Russell, Arthur Porges, Frederic [sic] Brown, and Robert F. Young, as well as the conclusion of Jack Sharkey's "The Crispin Affair" and Sam Moskowitz's profile of M. P. Shiel and H. F. Heard, "The Neglected Thinkers of SF."]