I thought I'd start this discussion here. What are some blink-and-you-miss-it moments from the movies you've noticed through the past 20 years of watching them countless of times? Mind you, I'm not talking about movie mistakes, but actual plot points added by the movies that are barely there.
I'll start with two. The first one, I actually only noticed about 2 days ago (which is why I wanted to open this discussion in the first place) and it's from Prisoner of Azkaban. Rowling has already highly praised Alfonso Cuarón on his understanding of her world, but this is a small something that means a very large something else. It's in the Marge dinner scene right in the beginning of the movie.
Notice that there is chair for Harry, but there isn't a seat for Harry at the Dursley's table. Obviously, Aunt Petunia, being the control freak she is, would never have a chair not matching the others in her dining room, but the table is perfectly set with 4 chairs around it with a fifth one (part of the set, that's the most ingenious aspect) is placed aside. We never see Harry actually using this chair, but all the Dursleys are happily sitting around the dinner table and there isn't any other vacant seat. It's a small thing that mostly goes unnoticed, but it carries so much meaning that it alone explains better the treatment Harry gets at the Dursleys than any other moment in the series.
Now, the second moment. This one I actually noticed about 11 years ago and was laughed at for pointing it out at a convention once.
In Goblet of Fire, when they arrive at the site for the Quidditch World Cup, you can briefly see this:
They quickly pass though the screen, from left to right. A wizard pulling two llamas with house-elves sitting on them. The point of this short scene is to show how differently Wizarding societies around the world behave and to see that there's a place where the wizard walks, the wizard pulls the llamas, for the elves to ride them! It is so fascinating and it makes me hate even more the fact they didn't include S.P.E.W. in the movie. You already had the perfect set-up: the knowledge that the slavery behaviour isn't widespread like people would think. I always love to imagine the possible plot ramifications we could have from these small details.
What about you guys? Any other blink-and-you-miss-it moments you've noticed?