It's harmful to give people caffeine they didn't ask for.
Max was waiting in a diner, and a customer wanted some decaf coffee, but there wasn't any brewed.
Max wanted to finish the table.
Max brews a fresh pot of decaf for the customer, pouring some into a mug for her.
Max grabs the pot of regular coffee, and fills up the mug of the customer without telling her.
The customer is satisfied, and gives Max a reasonable tip for her effort.
The customer is allergic to it and has a bad reaction, struggling on the floor.