When sliced bread was first sold, it was marketed as "the greatest thing since..." what?
The Cillicothe Baking Company originally marketed their Kleen Maid Sliced Bread as "the greatest forward step in the baking industry since bread was wrapped".
Apparently, before we found a way to slice bread on an industrial scale, simply bagging bread was the apex of human civilization. As to how the "greatest thing since sliced bread" meme got popular, blame Wonder Bread. The company started offering sliced bread nationally in 1930 and marketed the product using variations of that phrase, leading to an expression which is equally popular, misunderstood, and truly trivial.