It happened to me with The Professionals. A friend of mine sent me a link to a story, saying she thought I'd really like it since it was scifi. I did. A lot. It was an obvious AU and I didn't even know what one of the characters looked like, but I liked the writing so much I went on to read all the other Pros stories by the author. Then other Pros stories. I was hooked. Still am. But I did read the original story as original fic. Actually, I recently reread it and, even though I now know the characters as they're portrayed in the story are nothing like those on the show, I still love it. So I guess it's still original fic to me. One thing, though, it did make it hard for me to write in the fandom, since my impression of the characters isn't "true," not even after having seen the episodes, because it's overlaid with the one created just from the stories I read first. It's made it harder for me to create my own image of them.

Anyway, it did serve another purpose. Because of all the cross-overs between Pros and MFU, I eventually gravitated to MFU. That show I'd seen from its first airing, so the characters were already fixed in my mind. But it took reading them in a slash setting with Pros to get me into that part of the MFU fandom.