Quora question:

I always see this small black triangle on the inside of airplane walls. What does it mean or do?

And the answer:

The black triangle marks the location of what has been called “William Shatner’s Seat,” the seat with the clearest view of the wing. This is the place inside the airplane from which you can get the best visual check for ice or other problems. The Shatner reference is to one of the strangest Twilight Zone episodes, Nightmare at 20,000 Feet, which first aired on October 11, 1963. In it, Shatner’s character sees a gremlin on the wing of the plane he’s a passenger on.

Not sure if this is true, but I did a bit of lookup and there are a number of forums that tell a similar story. Great bit of trivia for your next flight, or the next time you meet William Shatner.