I remember playing a game about 15 years ago or so, but can't remember
the specific rules. The game involved a group of about five people,
and each was doing some activity (everybody did the same thing). The
goal of the game was to watch for when someone else stopped moving
(i.e., doing whatever it was that the game involved), and then
everyone else was also supposed to stop moving, and the last person
moving lost the game. The key, therefore, was paying attention, as a
lapse in attention meant that you would be the last person moving. I
don't think this was a commercial game, but it might have been. Or
perhaps it was a card game. Anyway, my question is: What are the
specific rules for this game? (Note: perhaps there are several games
like this, in which case a couple of variations would be nice).

The only similar game I know (but not quite the game you described) goes like this:
-1 person leaves the room
-the people in the room are sitting in a circle and they choose 1
person to be the leader
-the leader does something continuously (any movement) and everyone
else joins in and does the exact same thing
-the person who left the room then enters the room, their goal is to
determine who the leader is
-the leader occassionally switches to a new movement and everyone else
in the circle then switches to the new movement also

The game ends when the person correctly identifies the leader.