Some Tempest cards were printed on a press that hadn't been cleaned. For most of the cards, the residual ink is unnoticeable because it is so faint and is obscured by the business of the Magic card front.
But not all. R&D's Elaine Chase provided us with the two Medallions that have plain enough art for the residual press ink to be seen. There are faint cartoon characters visible in the background (most people agree they are from the comic Peanuts.)
While not particularly valuable, some collectors enjoy owning oddities such as these.