As a long time rider (no riding horses now) I can really laugh at this --- especially since in past I've watched MANY match ups at rental riding places!
Out West in Wyoming one year, on one of those 20people trail riding thru a national park, I was put on a small little gelding, maybe 13 hands. I was a small rider and had lots of experience, this little guy could get "spicy" and liked to move out. MY kind of horse!! All was going great and as we got to a rather large mud hole in the trail, one of the horses decided they wanted to roll in it! Rider was prob on first ride ever and didn't even know the pawing was a prelude to the next step --- LOL. Guides holler to pull up head, rider doesn't know how to....as the horse began to go to a knee, I kicked my spicey guy and we scooted in to stopped the swim!
LOL -- the sign says it all about the laughs for the trail guides.
MH, I had a horse roll on me once. She didn't paw or anything, just sunk to her knees. I jumped off and was scared something was wrong w/her-until she started to roll.