temporary image persistence in a 27inches iMac

Hello guys.

My iMac(27 inches, late 2012) had a strange issue 2-3 hours ago. This is the second machine I got, as the first machine I had received suffered from heavy image persistence and it was replaced.

So, I have had this second machine for six months, without a single problem of image persistence, working very often at it for 12-14 hours in a row.

This evening I was also working at my iMac , having a pdf opened at the left side of the screen for 5-6 hours. When , for some reason, rebooted my machine, I noticed some kind of image persistence with this document. After a couple minutes , it was gone , without really doing anything special.

From this time, I have opened at the same side of the screen for much time the same pdf, rebooted the system a couple of times to see if it will happen again, but nothing noticeable.

Should I really be worried about this fact or it could happened accidentally?

I want for no reason my iMac to be "opened" and repaired , especially the lcd screen , which is really the last part to get access inside the iMac. I think that this machine was manufactured to be as more compact as possible and do not want to get it opened or scratched at Apple reseller's service depar.

With regards,

Spyros.