What examples are there of scientific effects or correlations that we now know to be real, and were known about for a long time but thought to be coincidences?
To give an example of the kind of thing that I have in mind (but I don't necessarily know if these are real or the best examples):
People used to know that there was a relationship between the moon and the tides, but expert opinion was that this was just a superstition. Now we know the moon really does influence tides.
The "hot hand" effect in basketball. Originally this was claimed to be the result of random chance and a psychological bias to see patterns, later some papers claimed that this was a real effect. I'm not sure what the current state of the debate is.