Once when I boarded a flight I noticed one of those multi-country mains power sockets next to a 2-hole headphone socket. They were arranged and aligned in a way that made it obvious that a 2-prong headphone cable would fit in the power socket. I took a picture of it and put it on Facebook before taking off.

After a nap I actually did put on headphones and plug them into the power socket by mistake. Luckily nothing happened.