I just bought an iMac for my father for father's day and everything has been working great. However, today I noticed a long smudgy mark on the glass when the monitor was turned off. No problem, I got out the wiper and tried to wipe it off but nothing happened! It won't go away! You can only notice it when the monitor is off or if there is a lot of black like if you're looking at photos of space or something. Is this a common issue? You can use it fine, but I don't want my dad settling for something like this if it really is a defect. Has there been issues of LCD problem for the iMacs? Thanks.