When Max meets the bomber Guerity in O'Donnell's bar they both order a Guinness. The first slip-up is that they are given what looks like Coke, it has no sign of a white head on it!

After they have several sips and the glasses are half empty, Max leaves the room. The glasses on the bar then fill back up with Coke/Guinness and Guerity has another slip from a full glass.