https://en.wikipedia.org/wiki/Heegner_number#Almost_integers_and_Ramanujan.27s_constant

This concerns the number $e^{\pi\sqrt{163}}$, and says "In a 1975 April Fool article in Scientific American magazine,[7] "Mathematical Games" columnist Martin Gardner made the (hoax) claim that the number was in fact an integer, and that the Indian mathematical genius Srinivasa Ramanujan had predicted it—hence its name."

The punchline is tied into complex multiplication, though I don't know the details.