A Printing Error is a mistake made during the printing process that materially changes the card. Typically, these types errors impact very few cards today but in many releases before the modern era of card collecting printing errors happened regularly and often.
The most common printing error is to have a card come off the line off center or miscut completely. This typically reduces the value of the cards substantially.
A very common printing error from the 1940's and 1950's. The card fronts looks pristine, but the back is jst blank stock.
Cards with non-matching card fronts and backs.