Back in the olden days I was a pressman for a newspaper. We would grab the newspapers coming off the press, and hold them upside down to check the print quality, color registration, etc. This was almost necessary to see errors, not because you would be distracted by reading, but because you see what you expect to see. Turning the newspaper around changed those expectations, and allowed you to see errors.
Infantrymen on patrol are also taught to scan their eyes from right to left. I can save their life, by increasing the probability of spotting something unusual.
Yeah, when you are taught to draw, they always tell you to hold the picture up to a mirror. Any errors become blatant when you do that. Its also why you have to be careful when you draw at an angle when sitting down. As soon as you sit the picture up right once again errors show up where you didn't see them before.
In one particularly embarrassing example, I once spent dozens of hours on a project for a drawing class, only to find I’d included perspective distortion on the entire image from the angle at which I’d been viewing the canvas.
Luckily it turned out to be a good-looking effect, and I got a good grade by passing it off as intentional.
