My most significant "Eureka!" moment shortly after I joined SSC (more than a decade ago for that) or knew anything of a Tally/Numbers Table. It was when I first realized that a simple CROSS JOIN and a couple of ancient forumulas for constraining random numbers could be used to generate a million row test table at the drop of a hat (any hat :-D) with incredibly little effort and without any explicit loop (circa SQL Server 7). I used to call the method "set based loops" until a fellow by the name of R. Barry Young coined the phrase "pseudo cursor" for the method.

I certainly wasn't the first to do such a thing but I knew nothing, at the time, of those others that had made a similar discovery.