I can explain; It’s that form of minor panic you get when the person with the cash isn’t back by the time the last item is scanned and the total price is rung up. It can easily lead to a bit of awkward stalling the cashier for said person to return, especially if there are others in line and stuff already on the conveyor belt to the scanner.

Both my parents does that shit all the time. It's one of the few things that REALLY pisses me off do damn much. No matter how many times I tell them if they have not forgotten anything, or have everything ready, they pull that "Oh dammit! I forgot to get that ! Sorry, hun. I will be back very quick!". While everyone on the line and cashier glares at me like they are thinking of many ways to kill me.