Warning: If you don't know what a cue dot is, you probably
want to stop reading now. It's the curse of the cue dot: once you
have seen a cue dot, you are likely to keep seeing them whether you
like it or not.
A cue dot (also known as a change-over mark)
is a bright circle or oval that briefly shows up
in the upper right corner of multi-reel movies as an
indication to the projectionist that the reel is a few seconds
away from its end. The projectionist takes this as a cue to cut
over to the projector prepared with the next reel. When all goes
well, the audience doesn't notice that a reel change took place.
As cue dots only show up for a small fraction of a second,
most moviegoers never notice their existence.
However, once cue dots have been pointed out to you
and you've seen one, you're likely to keep seeing them,
whether you like it or not.
In the movie Fight Club, the character Tyler Durden
points out cue dots to the audience.