It was supposed to be a chivalrous moment.

On Wednesday, Queen Elizabeth II dropped by Canada House in London to celebrate the 150th anniversary of the Canadian Confederation. As she was arriving, Canada's governor general stepped in to help her on some stairs.

In the process, he grabbed her elbow. And while it might not look like a faux pas, touching the queen is a major breach of protocol. Though the Royal Family's official website says that there are no “obligatory codes of behavior” for how to behave, physical contact with the queen is a no-no. (Shaking hands is the exception.)

In a video clip, Governor General David Johnston can be seen touching the queen's arm as she glides down a set of stairs.

Johnston apologized for the mistake. “I'm certainly conscious of the protocol,” he told Canadian Broadcasting Corp. news. “I was just anxious to be sure there was no stumbling on the steps.”

He explained, too, that he shares the blame with that most nefarious villain: a rug. “It's a little bit awkward, that descent from Canada House to Trafalgar Square, and there was carpet that was a little slippy, and so I thought perhaps it was appropriate to breach protocol just to be sure that there was no stumble,” he said.

He's not the first to goof. In 2009, Michelle Obama raised some eyebrows when she embraced the monarch, a gesture that set the British media a-titter. The queen, for her part, didn't seem to mind. Buckingham Palace later put out a statement calling the gesture a “mutual and spontaneous display of affection and appreciation” and clarifying that there had been no breach of protocol.

In 1992, Australian Prime Minister Paul Keating put his arm around Elizabeth, earning himself the nickname the “Lizard of Oz” in British press. In 2004, French President Jacques Chirac appeared to touch the queen while she visited a Parisian market, prompting the Daily Mail to warn "Hands Off!"

On the off chance you find yourself in a meeting with the queen, the Royal Family's website offers this helpful guidance on how to greet her in the “traditional form”: “For men this is a neck bow (from the head only) whilst women do a small curtsy. Other people prefer simply to shake hands in the usual way.”