Wax sometimes stains checklists (and stickers too for that matter). This is melted wax that came off the wrapper when they heat-sealed it. Whether it is the front or the back depends on which side of the card faced the wrapper. This card is stained on the front. Sometimes they are stained on the back. Wax on the front of a checklist (or sticker) can easily be rubbed off with a nylon stocking (rub vigorously, being careful not to damage the corners).