Stigler's law of eponymy, as stated by Stephen Stigler, claims that discoveries are never named after their real discoverer. There are many examples, including:
Halley's comet, named after Edmond Halley, but discovered many centuries prior
Pythagorean theorem, known to Babylonian mathematicians long before Pythagoras
Venn diagrams, proposed by Christian Weise and Leonhard Euler, but
popularized by John Venn
Euler's number, the constant discovered by Jacob Bernoulli, but named after Euler (payback!!!)
Occam's razor, named after William of Ockham, but formulated long before his time (clearly, not the simplest solution here...)
Stiegler's law, which, according to Stigler himself, was originally discovered by sociologist Robert K. Merton!