To their credit, the layperson should know that 390 mg of caffeine is a shitload of caffeine. Most people don’t, unfortunately. The problem here wasn’t that it wasn’t clear how much caffeine was in the drink, but rather it wasn’t clear that there was caffeine in the drink, and there’s so damn much caffeine in it.
A few weeks ago my flatmate mentioned the caffeine pills he’d been using, said he had one every couple hours that day. “It’s like as much caffeine as a cup of coffee.” 200 mg per pill. He’d had four pills that day. Not medically dangerous since he’s in good health, but it still kinda scares me how people don’t take caffeine doses seriously.