It used to irritate me when people sat there at the window checking their orders, but only because I know I NEVER fucked up. EVER. Of course, they didn't know that, so I tried not to let it bother me too much. Sometimes I'd smile and say, "It's all there, trust me!"... but since fuckups are pretty common, I DO recommend everyone check their food - just not necessarily at the window while people are waiting behind you and the timer is running.
It's a $2 burger, it isn't as if they aren't going to give it to you if you come back five minutes later and say they forgot it.