It's embarrassing to fall in front of other people. It's extra embarrassing when your primary job description is "stand there and look serious."
A member of the Queen's Guard gave a bit of accidental flair to a recent Changing the Guard ceremony when he slipped and, to put it casually, "ate it." You can tell that the poor guy was pretty embarrassed too, blushing and looking side to side before settling back into his stoic norm.
Also, I love how the person who posted the video put in the title "falls in front of hundreds of tourists" as if tourists are the most-embarrassing people to do something in front of, and not the people who you are least likely to ever see again in your life.