“But it wasn’t stained when I brought it in”

Some stains caused by beverages, food, or oily substances may not be visible after they dry. But later, with exposure to heat or simply the passage of time, a yellow or brownish stain will appear. This is the end result of oxidation or caramelization of sugar or sweetening agents. It is the same process that makes a peeled apple turn brown after exposure to air. If we don’t know about it we can’t fix it, so let us know if you spilled something.