The possibilities of Photoshop are only limited by one's imagination. I don't know who originally had the idea to swap facial features in pictures of more than one person, but the results can be freaky! Sometimes the whole face is swapped; sometimes it's hard to tell where each part came from in this selection of images from the Something Awful forums posted at Unreality magazine. Link
The one on the left looks like William Windom (Commodore Matthew Decker) and the one on the right looks like Martin Landau (who was offered the role of Spock).
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