Before President Donald Trump met the Queen on Friday, anti-Trump protesters warned it would not turn out well.

Now that the meeting is over, they're counting the ways POTUS "insulted" the monarch. And of course the discussion is taking place on Trump's favourite platform, Twitter.

Trump tormentors – even self-described "non-royalists" – took to Twitter to acidly note moments during the tea time meeting at Windsor Castle when Trump did something that could be interpreted as a faux pas.

None of these moments were especially horrifying. He didn't grab her in a bear hug or trash her in a tweet as "fake news". But he did keep her waiting for more than 10 minutes (which may not have been his fault), shook her hand instead of bowing, turned his back on her for a few seconds, and he left his jacket open and flapping and his too-long tie trailing.

Many seized on the TV footage of Trump and the 92-year-old Queen preparing to review scarlet-clad troops assembled in the Quadrangle of Windsor Castle where they met. He turns his back on her, walks ahead and looks the other way as the diminutive monarch moves from one side to another behind him.

It is considered impolite to turn one's back to the monarch or walk in front of her. For the 70 years they have been married, her husband, Prince Philip, 97, has always walked a few paces behind her in public.

Even prime ministers, after their weekly audience with the monarch, back out of the room when leaving her presence. It's just the expected thing to do.

Nobody told Trump, or he forgot.