[scratt]

That sounds almost like the defect I thought I had on my brand new iPhone.. It was back in the day when they had that starburst third party menu system which showed a little white starburst with rays in one corner of the LCD which you touch to bring up all the icons... I forget it's name.. In the taxi after going through all the hassle of getting it I decided to just accept it.. FIrst adopter enthusiasm! It was really ugly and looked like the screen was cracked on the corner. I was super happy when I found out it was software!!

In your case that obviously is not the reason. What you have to ask yourself is why did the other guy bring it back?

Assuming it has the same guarantee I would definitely change it. It sounds like the panel has got crushed slightly and that the case is now not 100% square and you are getting some light leaking at the corners...

In any case the longevity of your backlight is now in question IMO.
If you can, change it.