I ran across the following (to me startling) example in Robert Cromie 1895 techno-thriller The Crack of Doom (reprinted in The End of the World: Classic Tales of Apocalyptic Science Fiction, Michael Kelehan, ed.)
Page 102: "If you consult a common text-book on the physics of the aether, you will find that one grain of matter, contains sufficient energy, if etherised, to raise a hundred thousand tons nearly two miles."
Here "grain" is a standard unit of jewelers (one gram = 15.4 grains). Then it is easy to verify, that within ±2% error, Cromie's "etherised" mass-energy relation is $E = m c^2/2$.
Einstein was 16 years old when Cromie's book appeared (published by a European publishing house) ... a very impressionable age, needless to say. Yet despite the clue that Cromie so generously provided to science fiction fans in Europe, ten years passed before Einstein got the factor of two right.