Taking your shoes off on a flight is considered a big social faux pas – and one you certainly wouldn't expect to see in first class.
Photos of bare feet resting on seats often end up on the Facebook page Passenger Shaming – but recently, three-time Olympic gold medalist Shaun White experienced it in person.
The snowboarder was travelling from Japan back to the US when he encountered a pair of bare feet resting on top of the seat behind in the first class cabin.
He posted a video of the incident on his Instagram page – slowly panning back to reveal the feet and then shaking his head in disapproval.
While taking your shoes and socks off on a plane is bad enough already, most travellers would agree that resting them on someone else's seatback is just not on.
Despite the fact that these feet were relatively clean compared to some of the revolting pairs posted on Passenger Shaming page, the extra room of the first class seats provided no consolation for the Snowboarder. Comments on the post agreed that this sort of behaviour was "gnarly" and "no bueno".
While it might seem a bit petty, going barefoot on a flight isn't just an etiquette issue. Planes can be breeding grounds for all kinds of nasty germs – and if things go horribly wrong, having no shoes on could be dangerous.
A ex flight attendant, Tony Kuna, told online forum Quora that having bare feet means you won't be properly protected in the event of an emergency.
"Besides stinking up the whole cabin, footwear is essential during an aeroplane emergency, even though it is not part of the flight safety information," he wrote.
"During an emergency, all sorts of debris and unpleasant ground surfaces will block your way towards the exit, as well as outside the aircraft.
"If your feet [aren't] properly covered, you'll have a hard time making your way to safety.
"Imagine destroying your bare feet as your run down the aisle covered with broken glass, fires and metal shards.
"Kind of like John McClane in Die Hard, but worse."
So keep those shoes on – or if you must remove them, at least wear slippers.