Yesterday I was working on a project in Adobe illustrator, that project had various white spaces it was then when I noticed a black spot/smudge in the lower corner of the display of my iMac. I thought it must be a dust particle on the outer glass screen of the iMac so I used a microfibre cloth to remove it which eventually failed cause it was then when I realised there is huge black smudge in the actual display of the iMac which is inside the outer glass screen
Gotcha! the black spot is actually dead pixels. I had an Apple Engineer to diagnose the display who said this is an obvious thing to happen and it doesn't happens because of daily usage. So I asked him why does this happens, he said that Apple has the tendency to reuse old and thrown out refurbished, recycled displays which is why sometimes these displays fitted into a brand new $1000 iMac shows these black spots/smudges specially in areas where the speakers are located!