I've recently bought a new cooling unit for my Core i5 760, and when I proceeded to remove the heatsink that was installed by the folks who assembled it when I first bought my computer I was surprised to see that there was a white sticker with a barcode on it covering about 1/3 of my cpu on the bottom side.
Looking through various unboxing videos of the i5 760 on YouTube (for example this video: watch?v=uFHXbsSPSVQ), I saw that the CPU had no stickers on it and it's surface was completely clear.
I have no Idea where this sticker came from then. I tried peeling it off but it was stuck on there pretty tight. So I gave up and just applied the thermal paste over it and installed the new heatsink.
However I'm getting some uneven results in terms of temperature on the cores. Core #0 and Core #2 are always about 5 degrees hotter than core #1 and core #3 (both with idle and full load). I can't help but wonder if this sticker might be the culprit because it's stuck right over Core #0 and core #2 and thus impeding proper heat dissipation through the heatsink?
I've searched online for uneven core temperatures with the i5 and there were some people who reported the exact same problem that I have, and I can't help but wonder if they also had a sticker covering 1/3 of their CPU.
It's fairly common and not really a problem, anyway. It just means your heatsink isn't 100% level, or there's a little more paste in between those cores and the heatsink. Unless the temps are borderline too high, I wouldn't worry in the least. Even if it is caused by the sticker, unless they're going to high, I wouldn't bother.