Ha! Something similar happend to me as well, in fact in relation to the series that I was wordcounting in an earlier response. It did start out as a serious story but it was just supposed to be one story.

Then I had a sequel. Then another. That kept up until I had a series of six stories. The original story was, I think, my second story so I totally had no reason to expect that anything like that might happen. Actually, most of my series happened that way; I wrote one story and next thing I knew I had a series. I've only planned out one series ahead of time. And OK, do I ramble on or what?