These days, almost every image that is used in marketing and promotion material gets photoshopped before. One of my favourite websites is Worth1000 which is a photoshopping contest website. The galleries contain a whole range of great photoshops. For example, check out the gallery of photoshops on life without electricity.
Today CyberNotes pointed out some photoshop mistakes. These include celebrities with three arms, headless people and hands with extra fingers. For example, check out the High School Musical 2 album cover.
There are a whole collection of photoshopping mistakes, errors and just general disasters at the Photoshop Disasters blog.