as you must have figured, I had the dubious luxury of seeing a shaking leg like that right now. Our meeting rooms are transparent glass, with shaded/glazed glass at the center leaving the top and bottom for clear viewing. So you can see the lights and anyone whose head is higher than 6 ft, and upto the knees of anybody in there. hence that sight. Which also reminds me of other things I have noted in the past.
Other things that I have seen sometimes are impatience. Women in specific seem to play with their shoes a lot, slipping in and out their feet. Especially in the case of sandals or open slippers.
With glass tables I think the tendency to shake legs under tables has reduced. But it still persists.
The funniest is when someone is trying to leave the meeting room, and another is detaining. the shoes can be seen to magnetise towards the exit door, while the other pair remains firmly rooted at a spot. the toes of the former then oscillate between facing the door and the other person.
Sometimes sitting at just one spot and noticing stuff does make for funny observations.