I've never had, or heard of it happening, to a MBP let alone an iMac.
Oh, there's the whole StainGate idiocy. Everything's gotta be a "gate". Yes, there was a problem with some run of MBP screens. Yes, Apple is replacing them.
I say idiocy because no matter what the problem might have been, the pictures the whiners were posting were clearly abuse.
Don't make it bigger than it is. Just because 1 product has it doesn't mean that any other product has it too. There is no correlation here. That is the point.