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When I switched my imac back on after Christmas I got some strange pixellation effect in the lower right hand corner. It looked cloudy and the resolution was poor. I was worried that my screen had suddenly taken ill so I rebooted into the Apple hardware test. When the computer was off I got a better view of said cloudyness. It looked like fogging underneath the screen, similar to what you get when you breathe on a cold window. See the lower right of the screen in the photo below.

A quick web search show that this is a known problem. The imac has a glass plate fixed to the front of the LCD screen. It appears that sudden dramatic temperature changes (e.g. leaving it in a room for a week with the heating switched off and then switching on the computer and the heater beside the computer at the same time) can cause condensation to appear on the inside of the glass plate.

The problem disappeared after leaving the computer running for about 15 minutes.

However if your impatient and don’t like the idea of condensation on your computer screen it’s actually very easy to remove the glass plate and wipe off the condensation. It is held in place with 4 small magnetic clips and you can remove it using a piece of tape or, eh, a plunger. See these 2 videos for an example of how to get the glass plate off. NOTE: removing the glass may void your warranty.