Maybe the milkshake counts as a "bottled drink," so that might be part of the glitch. I see people who finished the game - I guess they either won regardless or restarted the level.
The Gamezebo review of the game says this:
The most serious problem is a glitch that occasionally doesn’t let you deliver certain drinks to certain customers, resulting in the loss of that customer. This happened to me a total of four times over the course of the game, which isn’t enough to call the game unplayable – still, it’s an annoyance that shouldn’t be there.
Sorry I can't find out more.
