I know of a rather clumsy fellow in my dojo, he was demonstrating a technique with my sensei, and when he was thrown, he tripped over the sensei's hakama, bringing both of them to the floor
The student, needless to say, was embarrassed.
This has happened to me, but on a lesser scale, finding someone had stepped on my gi sleeve, preventing me to get up. He did not realize it, and was waiting for me to get up to start the technique again