86,243/Mistake

Source Work

 * The Dictionary
 * $2^{86,243} - 1$
 * $2^{86,243} - 1$

Mistake

 * ... the $28$th Mersenne prime, hunted down by on his trusty CRAY-1 in $1983$.

The date he actually found this was in fact $25$ September $1982$.

However, he delayed announcing this result until after he had officially received permission to actually begin the search (which, at that point, had already been under way for some time).