Earlier today, someone at the YUNiversity of Righteous Grammar posted this tweet:

A good rule of thumb — but, it turns out, not much more than that. Just a few hours later, I came across the word “prone” used to mean both prostrate and supine in Joseph Conrad’s Nostromo (Part 3, Chapter X).

Stuck on the Great Isabel, Martin Decoud, “the brilliant Costaguanero of the boulevards,” is about to die of solitude:

It had been a day of absolute silence — the first he had known in his life. And he had not slept a wink. Not for all these wakeful nights and the days of fighting, planning, talking; not for all that last night of danger and hard physical toil upon the gulf, had he been able to close his eyes for a moment. And yet from sunrise to sunset he had been lying prone on the ground, either on his back or on his face.

The OED defines the adjective prone as “situated or lying face downwards, or on the belly”; and my discovery of this exception to the rule proves only that:
A) Kory Stamper is right: prescriptivists can learn from descriptivists to be a little more accommodating;
B) good writers don’t always follow the rules, and sometimes make mistakes;
C) you never know how one thing you read will connect with another — a lesson in literary serendipity as ancient as the Virgilian dip; and
D) Conrad didn’t need the word “prone” in that sentence.