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T206CollectorWe're talking about wet sheet transfers, where the card appears to have been layed on the top of a wet version of itself leaving a stain on the back that looks somewhat like a reverse of the front.
If there was a factory spill of red ink at Factory 25 2d Dist Va in 1909, and all of the Juan Violat's printed that day have big red lines over the front of the card, that would certainly impact the value -- perhaps as much as your ball point pen example. It all depends on the size and nature of the mark, obviously.
My Cy Young portrait is a good example of a factory blemish causing a substantial hit to the grade, though centering didn't help on this issue either.