Joining the mile high club is something most airline passengers would never dare to do out of fear of getting caught - and it turns out it rarely goes unnoticed.
In a thread on Reddit, flight attendants from around the world revealed hilarious tales about catching couples in the act, and warned randy travellers they are not as sneaky as they think and they could be charged with a crime.
Cabin crew once shamed an amorous pair with a Champagne 'celebration' when they exited a germ-infested loo, while another couple was welcomed with laughter and cheers after a flight attendant made an announcement over the loudspeaker.
Reddit user Night-howl was working on a flight from the US to the UK when the crew realised a couple was joining the mile high club in a lavatory near business class.
It was around 2am and most passengers were asleep, but the flight attendants decided to shame the pair, who was British.
The Redditor wrote: 'We grabbed a bottle of Champagne, and about five of us stood outside the door and waited.
'When they came out, we cheered and clapped and drew quite a bit of attention to them. Popped Champagne, handed them a couple of glasses and warned them to bathe in bleach when they got home.'
Another user, highestmikeyouknow, who works for a US airline, was involved in a classic shaming while flying from San Diego to Portland.
He was putting a cart away when he noticed a couple exit the lavatory together and giggle as they returned to their seats.
He wrote: 'Without thinking, I grabbed the microphone and announced to the entire 737 "Ladies and gentlemen, please put your hands together for the two newest members of [the] mile high club!
'I was mid-sentence when I realized how stupid it was to say this... I wanted to reach into the air and put the words back into my stupid mouth.'
But the entire plane erupted into laughter and cheers as the woman hid her face in her hands and the man pumped his fist in the air.
A basket full of surprises
User lifesnotperfect used to work for Qantas, Australia's flag carrier, and said the moment she will never forget occurred on a flight from Australia to Vietnam.
Flight attendants put together a basket full of goodies for two newlyweds on the plane, but received a shock when a colleague delivered it to their seats.
The Reddit user wrote: "[She] goes down to their row with this nice basket with all the goods in it, and we're all watching from the flight attendant service/wait area, big smiles on our faces.
"She reaches their row, she turns to them, jumps, says something to them, bows a couple of times and walks briskly back towards the rest of us."
It turned out the flight attendant caught the pair in the middle of a sex act.
"We all teared up from having to hold our laughter in so hard," the user added.
Reddit user milehighthrowayyyy works as a pilot on short- to medium-haul flights and said there have been a couple of occasions where people have been caught.
One of those moments occurred on a two-hour flight to Greece, when a couple decided to get frisky in their seats.
He wrote: "Now that might be legit on a night time long haul flight. This was middle of the day. In summer."
Passengers alerted cabin crew and authorities escorted the couple off the plane when it landed in Greece.
'You probably have poop on you'
A former flight attendant who uses the Reddit name wanderlilly said she caught only one couple during her career, and she let them get away with it.
She wrote: "The rest of my crew were offended and wanted to knock on the door and ask them to leave, but did nothing about it."
She said she gave the man a grin when he emerged from the loo, and the woman who followed him out made an excuse about feeling ill.
She wrote: "I say go for it, have fun, have all the sex when and wherever you can, just know that half of the time those lavs are cleaned half a**ed at best.
"If you're getting all naked and sweaty in there, you probably have poop on you somewhere."
How often does it happen?
User beef_stewardess is a regional flight attendant and said it happens every now and then.
They wrote: "It's pretty obvious when people disappear one at a time... You see two empty seats where a flirty couple was sitting, and an occupied lav, and assume the guy isn't in there with the girl to help her through a bout of motion sickness.
"I generally don't care enough to do anything about it as long as they're discreet and don't stay in there for a year."
Can passengers get away with it?
A flight attendant who uses the Reddit name Kzzinn said 95 per cent of the time it's 'very obvious'.
They added: "I mean come on, sex in a public plane?! On large commercial planes it can go unnoticed, but as soon as people take their time in the toilets it can become a security hazard so we then watch out for those people and it soon becomes obvious what they've been up to."
How dirty is the loo?
Reddit user Night-howl said joining the mile high club 'is one of the least sexy things you can do in life'.
They wrote: "Subway floors are likely cleaner."
Another user, highestmikeyouknow, warned travellers "don't even try it".
He wrote: "The bathrooms are disgusting cesspools of scum and filth which rarely get cleaned.
"Seats, tray tables, seat belts, the floor, carpet... pretty much everything on a plane is really dirty."
A locked door can be opened from the outside...
Most passengers probably don't know that flight attendants - or anyone with the know-how - can open a locked lavatory door from the outside without a tool or key.
User BowieBlueEye, a former flight attendant from Britain, said they have never understood the fascination with the mile high club.
They wrote: "Yes, we usually know. The toilets tend to be right next to the galley and yes we can open the door from the outside very easily if need be."