Justin Herrera had a John Doe problem. Using the traditional placeholder, he sued three John Doe defendants on time. But by the time he obtained their names and updated the complaint for injuries he suffered while locked up in the Cook County jail, the statute of limitations had expired and the defendants, three correctional officers, attacked the lawsuit as tardy. The crucial question was whether Herrera made the kind of “mistake” that would permit the amended pleading to relates back to the date of the initial filing …