I shot a short video of this since it was easier than trying to describe it. It started happening recently but since I don't restart my iMac very often I don't know excaclty when. Besides that the computer seems normal, is it something to be worried about or might it dissapear with some future software update?
http://www.youtube.com/watch?v=DZ3YcH3lm_U&list=UUyY1dayEwjTzp0yKNj-411Q&index=1
http://www.youtube.com/watch?v=DZ3YcH3lm_U&list=UUyY1dayEwjTzp0yKNj-411Q&index=1