In 1909, Cambridge undergraduate G.I. Taylor published a letter describing his observation of diffraction using light of an extremely low intensity. For years, I've been teaching my students the interpretation of this experiment as demonstrating certain core features of the quantum theory of light. The story was that Taylor did this to prove that a single quantum could be diffracted. I recently learned that this was wrong on both historical and physics grounds.
The actual paper never even mentions quanta. It cites no references, but the only theoretical motivation given is from an obscure theoretical suggestion by Thomson.
Also, it turns out that, for reasons that could not possibly have been anticipated in 1909, the experimental conditions did not really test this aspect of quantum mechanics; the first confirmation of such an effect that is now believed to have been valid was by Grangier et al. in 1986. (Grangier, Roger, and Aspect, "Experimental evidence for a photon anticorrelation effect on a beamsplitter," Europhys. Lett. 1 (1986) 173. Paywalled.)
There apparently was some point in history after 1909 and before 1986 when people did interpret Taylor's experiment as a correct test of quantum mechanics, with a positive result. When was this? Did it play any decisive role in the debate, in which Bohr's followers wanted to quantize the atom but keep the EM field classical?