I had this problem about 18 months ago. A little bug inside my 20" Cinema Display.
Like a fool I pressed on the screen to get rid of it, only splattering it and leaving a horrible mark near the centre of the screen, DOH!
Now that monitor is resigned to the accounts office.....
Here's a pic, i've highlighted the splattered remains of the bug that have marked the inside of the screen as it's hard to see with my iphone picture.