Angels Cast Reflections?
There is a scene where Nicolas Cage (Seth) is standing next to Meg Ryan (Maggie) as she looks into a mirror. Because Seth is an angel he has no reflection. However, just before this shot there is another in which Seth is standing slightly to one side of Maggie. In this scene, Seth is clearly visible in the mirror.
When Maggie is in bed with the book from the library, she can't sleep and starts to read. Before she rolls over to get it she is in a tank top. After she grabs the book she has spaghetti straps instead.
The first scene in the kitchen between Seth and Maggie, if you look at the time shown on the clock on the stove and on the wall, you'll notice it changes dramatically depending on the camera take.