Psycho - 1960
Of course, I've seen it a few times already but wanted to see it on blu-ray.
I noticed something I had never paid any attention to before. After the opening title sequence, the camera is panning the city and then the words "Phoenix, Arizona - December 11, Friday 2:43 pm" appear. I started to wonder why the date was so specific and if it was Dec. 11, shouldn't there be Christmas decorations in some of the scenes? I watched the movie looking for some sign of Christmas and didn't notice anything making me very curious as to why that specific date was chosen in the opening scene. I re-watched the beginning and turned on the commentary to see if the commentator would mention it. Turns out that in one scene where Marion is leaving town, there are Christmas decorations on the street lights (I just didn't notice them). When they were shooting the scene, they didn't realize it and putting the Christmas decorations in the movie was not intentional and when Hitchcock noticed them, he inserted the date at the beginning to cover the flub.