When Coke Zero marketers invited Web browsers to “take advantage of our revolutionary Sue-A-Friend Technology and sue the pants off anyone who is copying you,” they weren’t really soliciting suits against the Coca-Cola Co.

But the diet drink’s ad campaign — featuring Coke managers who want to sue their counterparts at Coke Zero for “taste infringement” — has taken a sharp turn from camp to the real thing in federal court in Atlanta.