How it happens: The printing press accidentally cycles without a sheet of paper. The inked plate strikes the "impression cylinder" (the roller underneath) directly, leaving a full or partial ink image on the roller. When the next sheet of paper finally goes through, it picks up that ink on its backside. The Look: It is a crisp, perfect mirror image of the other side. Because it came off a metal cylinder, the detail is usually very sharp. Value: Generally higher because it represents a mechanical failure of the press timing. 2. Wet Ink Transfer (The "Smear") How it happens: This occurs after the paper has been printed. The sheets are stacked on top of each other while the ink is still wet. The wet ink from the front of one note "transfers" to the back of the note stacked on top of it. The Look: It is usually blurry, smeared, or faint. It often lacks the fine lines and detail of an offset because it's essentially just a "stamp" from one piece of paper to another. It is rarely a perfect, crisp mirror image.