Carl is a Mistake not in the sense that he surprised his parents by being conceived, but in the sense that he is accidentally immortal and threatens to disrupt the Cosmic Swing of Things, ushering the era of Divine Intermission.
It is for this reason that he has been hurried to another reality in order to meet a retired deity. With the proper help, he might be able to set the universe right.
But first, he must survive his time on the Infinite Beach, where he may have garnered the attention of something much more sinister than an impending cosmic cataclysm...