Analytics revealed to me that a product I'd built (well, a free side project, but that's beside the point) was succeeding in the exact opposite of the market I'd built it for.

I used to run an online Japanese-English dictionary. It was geared towards people learning Japanese. I built it when I was studying Japanese and thought it would be useful for other students.

Turns out the overwhelming majority of the traffic to that site was from... Japan! I had it completely backwards! People weren't using it to aid their Japanese studies; they were using it to aid their English studies. (You might be tempted to suggest that it was in fact being used by people learning Japanese, and that those people were taking part in exchange/immersion programs, but there were other data that disproved this hypothesis.)

I never would've realized that if I hadn't had analytics on there.