The Simon Effect is one of those interesting cognitive phenomenon. Here's the definition by CogLab:

The Simon effect refers to the finding that people are faster and more accurate responding to stimuli that occur in the same relative location as the response, even though the location information is irrelevant to the actual task (Simon, 1969). Studying the Simon effect gives us insight into a stage of decision making called "response selection." According to information processing theory, there are three stages of decision making: Stimulus identification, response selection and response execution or the motor stage.

More about it: Simon, J.R. (1969). Reactions toward the source of stimulation. Journal of Experimental Psychology, 81, 174-176.