I try to look for errors in continuity. Which is, when you shoot a scene, you have to make sure everything is exactly the same. This is continuity.
I remember one film, I saw a long time ago. It was a romance film, but I can't remember what it was called. Well, the couple were in a French cafe, and they're eating croissants. It's a wide shot of the area around the table, and everything's fine. There is a zoom on the man's face, and he's talking, same with the woman, then it zooms back out and look! The croissants have turned into pancakes!