I got my new iMac 27" late 2013 yesterday. I ordered it direct from Apple online store, so I was surprised to see that it came with Mountain Lion preinstalled. I upgraded to Mavericks and today I saw a vertical purple line, almost in the centre of the screen. The line appears even when the Mac starts, so I guess it can't have anything to do with Mavericks... Apple is sending me a replacement Mac and I guess it is a hardware issue, I just wanted to ask you people, if you have installed Mavericks on your late 2013 iMac 27" (with the 1GB NVIDIA card) and if you are experiencing any issues. I am thinking of trying to downgrade, just to see if the problem persists, but it doesn't really make any sense. NVRAM and SMC reset haven't helped either. The hardware test didn't find any problems.