Yes, people who have been on the Guard are trained to keep their knees unlocked. But, after long standing, some people’s minds wander and they instinctively lock their knees. That’s when even seasoned Guardsmen mess up. I have actually seen a guy fall asleep standing up and not fall down. As the Commander, I had to subtly wake them up. Usually, for super long winded political speeches, I would snap around and do an “inspection” walk to each member and insure they were okay. If a member was stressed, I would have them do an individual fall out movement and go back stage. Then they would come back after a free minutes. We made it look as professional as possible.