Probably the greatest academic 'mic drop' in history: In 1903, Frank Nelson Cole of the Columbia University delivered a "talk" with the title 'On the Factorization of Large Numbers'.
Apparently he, silently, wrote the following on the blackboard:
2^67 - 1 = 147,573,952,589,676,412,927 = 7,618,383,257,287 x 193,707,721
and then walked out (or something).
p. 183 of Singh "Number Theory step by step".
(2^n -1 was one of a list of numbers Mersenne had claimed to be prime).