Reminds me of one New Years Eve party where Aliens was playing on the TV and a friend walks past and says "Watch Bishop's severed body now as he grabs Newt to stop her getting sucked out the air lock...", and clear as day you can see the lower part of the actors body come out a hole in the ground when he grabs Newt. I had probably seen the movie a dozen times before but never noticed it.

In a similar vein, just after having watched X-Men 2 in a cinema a (different) friend who did CGI animation for a living pointed out that during the scene where Magneto moved the bridge, you can see that the shadows on the water don't move with the bridge. His theory was that the special effects team were likely completely aware, but looked at the amount of effort required and hoped no one would notice. I never noticed the problem myself until it was pointed out.

I guess back then when special effects were monumentally difficult and computers weren't as capable as they are now, directors often relied on the audience's attention being elsewhere at that moment and thus being less likely to notice.