While fixing up a scan of an old photograph for use in a book, I used a graphics program to simply paint some black over various white spots and streaks in the otherwise solid-black background.
Later on, I printed a couple of pages on a color laser printer simply to compare a few different type fonts.
I was horrified to see that the photo that had looked perfect on my LCD monitor, had dark black blotches against a grayer background.
It was a scary and valuable lesson, and I'm glad I learned it before the book was printed. Apparently most LCD monitors just don't have the ability to display the full range of colors that can be printed -- or even the colors that can be displayed by an ancient CRT monitor.
I re-did my retouching.