I found the following story in Robin Lane Fox’s The Classical World. He’s quoting from Aulus Gellius’s Attic Nights, who in turn is quoting Favorinus:
Lucius Veratius was an extremely wicked man of immense brutality. He used to consider it very amusing to slap the face of a free man with the palm of his hand. A slave used to follow him, carrying a purse full of change, and whenever he had slapped someone, he would order twenty-five small coins (asses) to be counted out, as prescribed by the Twelve Tables. As a result, the praetors later decided that this law in the Tables was obsolete and defunct, and declared by edict that they would appoint assessors to estimate personal damages instead.