What Harris is getting @ is this: when you see a movie in a theatre, the projection booth is high above your head and angled down @ the screen, creating a trapezoidal image. A decent theatre will file the aperture plate to compensate for this so that the image displayed is a rectangle. Filmmakers know this and build a certain amount of give into their framing so nothing will get cut off.
However, some people have become so obsessive about the film being shown in the proper dimensions (the correct numerical ratio) that some transfers have actually cut off image information in order to appease these people. This is what pisses Harris (and myself, for that matter) off.
Here's a link to some good examples: Letterbox Lunacy!