This always happens at least once when I am elbows deep on bar, so I am not paying attention to who orders what at the register. Someone will be hovering, and tell me "oh can you make that with extra whip/topping/I wanted that with heavy cream/something that you weren't doing to the drink". I'll fix it and make it what they asked, assuming it was their drink. Then when I put it on the bar and call out a name, someone else comes to get the drink and I find out the person telling me how to make the drink wasn't even the owner of the drink....