I usually begin a column with an essay of about 800 words on a topic related to science fiction, generally a look at a specific type of sf. Then I review 7-8 books. Often I write the whole thing from beginning to end, but sometimes I write the different bits out of order.
In this particular column, I left the intro essay until last. Partly that was because I was still doing some research on the topic I'd chosen. So there I was, all the reviews done and ready to launch into my intro.
I organized my notes, then started into the intro. It was in interesting topic, and I had some definite opinions I wanted to work in, so I was looking forward to it.
Then something weird happened. Five or six paragraphs in, what I was writing started to seem familiar. With about one-third done, I couldn't shake a nagging suspicion. So I took a break and searched through my old columns.
You're way ahead of me, I'm sure. I had covered the same topic, in just about the same words, less than two years ago. Now, I have no compunctions about revisiting a topic if I have new things to say—but in this case, I didn't.
Needless to say, I spent the rest of the day picking a new topic, researching it, and writing my essay. (For the record, this new topic is sequels to classic works, and I think I said some cogent things.)
So, happy ending. For a moment, though, I was convinced that my memory was shot. Not a wonderful feeling.