I've only done it once before. I had an issue of Fantasy Empire that I bought new because it had an article about the new Doctor (Peter Davison at the time) in it. I tore out the article and threw it in my collection of fannish stuff, and tossed the remainder of the magazine.

Years later, I was looking at that same article and noticed that on the reverse of the last page was the first page of an article about a show I had never heard of at the time but had become a big fan of since, Sapphire & Steel. I had to hit up the used magazine bins at SF/Fantasy cons until I found another copy of the same issue I'd tossed so I could read the Sapphire & Steel article.