Thanks for reminding me and for posting the link originally, Muirann. This is exactly what I was referring to. It's a long but very informative video. It doesn't explain everything we need to know, but gives an idea of the 'photoshop-ing' stuff...... Muirann's comment with her original post is also illuminating....

The experience with the mixed up scan shows exactly how human error can creep in, the more removed the process the more likely it could happen, more chance of error creeping in, either way.