A film platter is a monstrous looking thing:
http://www.flickr.com/photos/37769587@N06/3827414396/
On this photo, the movie pays out from the middle deck and rewinds on the lower deck. The upper deck is for backup or, as in the picture, holding a second movie.
Look at the bottom (rewind) deck. See how the blue film (actually the leader) crosses the platter as it rewinds on the center spool?
If the operator misthreads the film and it drags across that metal platter on its way to be rewound, it will get crosswise or diagonal scratches in it. It looks like a cat clawed the screen with hundreds of little, black scratches.
Vertical scratches usually occur as the film drags across a stationary object, lengthwise. Most often this occurs when the operator misses a roller, causing the film to rub the projector frame as it enters, on its way to being projected.
The biggest problems occur because a lot of scratching and damage occur AFTER the film has been projected and is on its way back to the rewind spool. Any damage that occurs then will not be seen until the next time the film is projected.
The number one way to prevent all of this is to have the operators walking the projection room, visiting each projector at least once every 10-20 minutes. Instead, what they do is haphazardly thread, press the start button then go downstairs and chat up the chicks at the popcorn counter.
BTW... Policar;
When I am on my game, I can build a 6-reel (2 hour) movie onto a platter and have it ready to play in approx. 90 minutes. 45 minutes after the movie is over, it can be back in the can. Myself and one other competent person can reshuffle all the prints from theater to theater in a 20-plex theater in an hour or less.
I am not exceptional. It just takes somebody willing to do the work. The problem is that fewer people, these days, are willing to do it.