I have no idea why Air Canada (or whomever designs for them) thought to place the controls for seat-back entertainment right where the human elbow rests. You end up turning off your own TV, as well as that of your neighbor, whenever you try to rest your arm.
This happened to me more than once during a single flight.
Oh, Air Canada. We may be stuck in coach, but we generally have arms; and those arms have elbows.
Testing this would not have been too hard. Just make stickers, place them on coach-class seats, and ask a few people to try them. This, however, involves thinking like a passenger, so it apparently wasn’t done.