When splinching occurs, a part (or parts) of the body can been left behind, and are missing when the wizard arrives at their destination. Arthur Weasley tells Harry that he knows of a pair that Apparated without a licence who splinched themselves leaving half of themselves behind and becoming stuck when they appeared in their new location. The Accidental Magic Reversal Squad deals with people who have splinched themselves.
Splinching, the separation of random body parts, occurs when the mind of the person Apparating is insufficiently determined. Susan Bones splinches herself during the Apparition lesson in the Great Hall, her left leg standing five feet away from where she started.