LeBron James apparently committed a faux pas last night when he slung an arm around Kate Middleton at a photocall after a basketball game attended by the Duke and Duchess of Cambridge in New York.
The NBA star - a huge name in the State-side sporting world - clearly treated Kate in the way he would any other of his thousands of fans. But her slightly uncomfortable pose suggests the infamous "no-touch" rule for the royals does indeed exist, and was breached.
If LeBron transgressed this archaic protocol, he's not the only public figure to have done so.
The world of royal etiquette is populated by a mass of vague customs and unwritten rules, making it notoriously tricky to navigate.
In part, this draws from tradition.
"From medieval times, monarchs were divinely appointed to rule by God, so they were kind of seen as gods, so they demanded to be treated as gods," Dr Kate Williams, a historian at London's Royal Holloway university, tells the BBC. "They are treated as people set apart from the rest of us, so primarily what it is creating is distance and grandeur."
Royal protocol is also a matter of logistics, according to David Miller, director of Debrett's (the go-to bible of British etiquette).
"It's wrapped up in history and tradition, but it's also practical, universal and there to avoid embarrassment," he says.
So what are the rules, where do they come from and who breaks them? We take a closer look...
To touch or not to touch?
Back in 2009, US First Lady Michelle Obama and the Queen astonished bystanders by sharing an impromptu embrace at a glitzy Buckingham Palace reception ahead of a G-20 conference.
Such was the public reaction at the event - aides claim to have never seen Her Majesty indulge in such a gesture in over fifty years of public service - that a Buckingham Palace spokesman was forced to issue a statement saying that there was no ban on being tactile with the monarch.
"It was a mutual and spontaneous display of affection," he said. "We don't issue instructions on not touching the Queen."
When a shop manager appeared to guide the Queen through a fish market in East Sussex last year, a rep denied that it was a problem.
"She did not touch the Queen, and even if she had done the Queen would have taken no offence," a spokesperson said.
Nevertheless, it does seem that touching the royals is pretty much off-bounds. Whether by directive or convention, most people restrict themselves to a handshake as opposed to a hearty back slap or bear hug.
Former Australian premier Paul Keating felt the wrath of public opinion and was dubbed the "Lizard of Oz" for touching the Queen's back during a royal tour to Australia in 1992, in what was seen as a purposeful snub and a representation of his country's republican desires.
Breaking bread with the royals
The concept of food and the royals is historically a bit fraught. According to newspaper memos leaked last year, Her Majesty got irate with her royal protection officers when they kept eating her private stash of Bombay Mix (who can blame her?)
If you're actually invited to eat with the royals - as opposed to just nicking their snacks - the rule of thumb is to follow your host's lead. So if the Queen is standing, you too should be standing (unless she's giving a speech). Wait until she is seated and then take your place. Take all your cues from her, or whoever the ranking royal is at the event in question.
Paul Burrell, former royal butler, recalls a particularly awkward incident on the Royal Yacht Britannia in the South Pacific, where a local prince forgot to take the lead from the Queen.
"The prince forgot to watch what the Queen did - instead, he popped the grapes into his finger bowl, then some cherries, then when the cream and sugar came out, he poured them in too, making a kind of fruit soup," he says. "I was standing behind the Queen looking horrified. He was about to raise the bowl to his lips to drink it when he looked at the Queen and realised he had made a terrible mistake. Not wanting to make him feel awkward, she picked up her finger bowl and took a sip. Now that's class."
You're also supposed to stop eating when the Queen stops eating, a rule that dates back centuries in the British monarchy.
It made life difficult for guests of Queen Victoria, who was an infamously swift eater (she could get through seven courses in half an hour).
"For many people eating with her was purgatory," according to an entry on Etiquipedia. "Everyone was served after the Queen and when she had finished all the plates were cleared for the next course. If you were the last person served often you wouldn't get a chance to eat anything before your plate was taken."
Still, at least this was better than the court of King Henry VIII, who was known to throw sugar plums at his guests when he got bored or frustrated with dinner conversation.
The curtsy question
Confusingly, the official website for the British Monarchy states "there are no obligatory codes of behaviour when meeting The Queen or a member of the Royal Family" before passively adding, "many people wish to observe the traditional forms."
"For men this is a neck bow (from the head only) whilst women do a small curtsy," it says. "Other people prefer simply to shake hands in the usual way."
If you're not British, you can take it easy - you're not expected to curtsy, as the queen is not your head of state. And the situation in general has relaxed considerably since the Queen first came to the throne in 1952.
"I don't think that they are as hot on etiquette as most people think they are," says former royal correspondent Jennie Bond. "They like people to curtsy, but you're always told at royal briefings that it's up to you. As a journalist, I never did. All this thing about not speaking to the Queen unless you're spoken to, I don't believe that, I always used to tell her jokes."
This wouldn't have cut the mustard in the Tudor court, where female and male subjects alike received a considerable calf workout by holding a low bow or curtsy until the royal couple of the time passed them (however long that took).
They were also expected to bow or curtsy an exhausting three times before leaving royal company - and the more flexible of courtiers were invited to "do this whilst walking backwards but, whatever you do, don't ever turn your back on the King."
Ma'am, Her Maj or Liz; correct forms of address
The terms needed to address the royal family are something of a minefield, divided into first and second references.
Apparently, when you first meet the queen you should address her as "Your Majesty" and subsequently "Ma'am" (rhymes with jam, not arm).
For other female members of the royal family, you are supposed to use "Your Royal Highness", followed by "Ma'am" in later conversation. And for male royals, it's "Your Royal Highness" and subsequently "Sir".
However this might be a bit too much pomp and circumstance for Prince William, who once remarked, "I am and always will be an HRH. But out of personal choice I like to be called William because that is my name and I want people to call me William – for now."
Typically, British monarchs have been a bit more touchy about their titles over the course of history.
Medieval king Edward II was always referred to in letters and writs with the rather wordy but impressive, "Edward by the grace of God King of England, Lord of Ireland and Duke of Aquitaine."
Inviting the royals to a party
If you thought merely addressing the royals was complicated, try writing to them with an invite. Let's just say, it involves quite a lot of jumping through hoops.
Debrett's suggests first making "an informal enquiry to the relevant private secretary or lord-lieutenant", to work out whether your invitation would be looked on favourably.
Then when you have the go-ahead for a formal letter, you should work out which royal family members to invite - bearing in mind that it's massively bad form to invite two or more members of the Royal Family (not including spouses) to the same event.
If the Queen decides to accept your invitation, you need to title all subsequent invites, "In the gracious presence of Her Majesty The Queen", at the top. In the case of all royal guests, you need to work out the name of the equerry or lady-in-waiting who will be accompanying them to the event.
The Queen has known to put in a spontaneous appearance to events she is invited to - like this couple she surprised at their wedding in Manchester in 2012 (they had previously written a "light-hearted" note inviting the monarch to their nuptials, but did not expect her to attend).
If the Queen happens to invite you to an event, you should apparently view it as a command (so no sloping off early to drink gin in front of the TV!).
Making small talk
Apparently, the Queen is - well, queen of small talk. It is, after all, a skill she has honed over decades of public functions with popes, presidents, pop stars and more. She likes to start with the opener, "Have you come far?"
This account of her meeting with Pope Benedict in 2010 shows just how much effort she invests into putting her guests at ease. She peppers him with light questions about his mode of transport: "It was a very small car you arrived in, wasn't it? Very tight squeeze?... But you have got your own car here, haven't you? The popemobile?"
You should keep your chats with the Queen easy and neutral, avoiding any political or religious flash-points. If she should happen to make a slip by confiding in you some contentious personal opinion (like her aside to BBC security correspondent Frank Gardner about the radical cleric Abu Hamza), don't whatever you do go public with it.
Keep an eye on what the Queen does with her handbag; apparently she uses it to indicate to her ladies-in-waiting that she is tiring of speaking to someone and is ready to move on.
The Royal Family has always embraced dogs. From the 17th Century onwards, British monarchs were frequently pictured alongside their pets, from pugs to greyhounds and King Charles Spaniels.
The Queen's favourites are, of course, her beloved corgis. She currently has two - Holly and Willow - along with her two Dorgis (cross-breed of Dachshund and Corgi); Candy and Vulcan.
It's reported that the monarch has banned Kate and Wills from bringing their black cocker spaniel Lupo to her Sandringham residence, over fears he will get into a scrap with her adored pooches.
Naturally if you want to stay on the right side of the Queen, you have to admire her corgis - but this is probably best done from afar.
Once at an informal lunch, a guest trying to pet them was reportedly rebuked with the words, "Leave them alone please. They are my dogs, they don’t like other people petting them."
One of the footmen assigned to the royal household commented, "They’re yappy, snappy and we bloody well hate them... for some reason the Queen will not allow them to be fully house-trained."
Royal staff are said to be armed with blotting paper for mopping up accidents and soda siphons, for squirting to get yapping dogs off ankles.
The Queen herself even needed three stitches to her hand in 1991, after breaking up a dog fight. So if you do happen to come across the corgis, play safe and make appreciative noises from a certain distance.
What do you think? Is royal etiquette completely outdated or worth paying attention to for tradition's sake? Let us know your thoughts in the comments section below
Images: Rex Features and Getty Images, words: Anna Brech