This is quite a good post from Oddee. It shows some examples of Photoshop mistakes where the user has not looked at the bigger picture. In one example a person has been "photoshopped" out of the picture but their hand has been left in place on the girls shoulder, whilst in another a basketball player seems to have lost his head. It can be easy to get caught up in Photoshop and forget about the overall look of what you are trying to achieve, let these examples be a warning.