Ron gets splinched when he apparates away from the Ministry after taking the Slytherin locket from Dolores Umbridge. Since Ron had not passed the test for apparition, it is possible that he may not have had sufficient practice to apparate. This led to him getting splinched and leaving behind a chunk of his arm back in the Ministry. However, Hermione's quick thinking and treatment with the Essence of Dittany helped stabilise his wound and regenerate his arm.