Everyone gets their hands soiled at the toilet on occasion - no matter how careful you are. Imagine you have drunk friends over, or careless neighbors, or even small children. And suppose one of them had their rare hand soiling experiences. Now suppose you had the paper under touching the wall behind. This soiled hand reaches for the TP, brushing the wall, leaving a brown, yellow, or possibly white residue behind. Now imagine you are the next person to reach under for some paper...
Over for me!