This is apparently a true story although it reads as a joke would.

When Ronald Reagan was president, he was either visiting or being visited by Queen Elizabeth. They and some others were horseback riding when the Queen's horse suddenly and loudly passed gas. Embarrassed, the Queen said, "I'm so sorry, Mr. President."
Reagan replied, "If you hadn't said anything, I'd have sworn it was the horse."

Maybe someone can validate the story. I recall the Queen once visiting him in California.