Just thought I'd post a heads up on here. It's that time of year when it's starting to get colder so there's a higher chance of this happening. A couple of weekends, I walked into the study and noticed a huge, misty mark underneath my screen. I'm pretty meticulous about keeping it clean, so I knew it was nothing I'd done and, after a while, it started to shrink. Someone, condensation had gotten inside of the screen. I have AppleCare, so I gave Apple a call and they advised that this shouldn't happen. iMac screen should be air tight, so this problem should happen. I think pretty much anyone is within their right to request a repair/replacement when this happens but, to be honest, it's a bit of an inconvenience as it would require a lot of backing up and manual labour (those iMacs aren't light and they don't carry themselves, you know?). An alternative approach I was offered was to leave the iMac on for around 24 hours, to burn off any excess condensation left in there, under the premise that I should call back and arrange a replacement/repair if it occurs again. It's been nine days now and, so far, I haven't seen any evidence of condensation. I know that still means the screen may not be completely air tight, but if it's not going to cause major problems then I don't want to make a mountain out of a molehill. So, I'd just thought I'd share in case anyone has similar issues. For the record, my iMac is a 2011 27". Also, if you opt for the 24-hour method, make sure that you still call Apple. That way, you have a record of the problem on their systems which will save a lot of explaining and hassle should the problem occur again.