The Queen did a double take when she saw her daughter, the Princess Royal, waiting at the end of a line-up at Buckingham Palace's Nato reception.
After shaking hands and chatting with US President Donald Trump and First Lady Melania, the monarch turned her head to see which world leader was next to greet, but spotted Anne waiting at a distance instead.
The princess, raising both her hands in the air, laughed and remarked "It's just me", before adding "and this lot", pointing to the members of the household behind her.
In video footage of the reception, Anne turned to point to the Deputy Master of the Household, Lieutenant Colonel Anthony Charles Richards, and Lord Chamberlain Earl Peel - the most senior official of the Royal Household.
Twitter users speculated that the Queen was chastising her daughter for not greeting Mr Trump, and that the princess responded with a shrug.
But Anne was standing back as she was not part of the official welcoming party, and the monarch was looking to see which politician was next.
Mr Trump was the last of the Nato leaders to be greeted by the Queen, the Prince of Wales and the Duchess of Cornwall in the palace's Music Room.
At the following royal drinks reception in the Green Drawing Room, Anne was also present when the controversial billionaire-turned-politician appeared to be mocked by other leaders.
A huddle which included Prime Minister Boris Johnson and the princess was filmed apparently gossiping about the US president's unorthodox style on Tuesday night, although Mr Trump was not mentioned by name.
During the casual chat, as the leaders sip drinks, Mr Johnson can be heard asking French President Emmanuel Macron: "Is that why you were late?"
Canadian Prime Minister Justin Trudeau says: "He was late because he takes a 40-minute press conference off the top."
Later in the conversation Mr Trudeau adds: "You just watched his team's jaws drop to the floor."
A video clip of the chat, filmed as world leaders met ahead of a summit to mark Nato's 70th anniversary, has been viewed almost four million times since it was posted online.
It came after Mr Trump branded Mr Macron's comments that Nato is "brain dead" as "very nasty", before the pair met on Tuesday at the US Ambassador's residence in London.
Mr Trump also pressed Mr Trudeau on Canada's Nato spending at their bilateral meeting.
Meanwhile, following an evening reception hosted by Mr Johnson at Number 10, a member of the US Secret Service was pictured leaving with a Christmas bauble given to leaders.
Each Nato leader was presented with one of the decorations and invited to hang it from the Downing Street tree.