For anything where a cash register or money transaction is brought into play I find that people are far more likely to slow down the line and make the person at the register want to head butt them by not being able to remember the pin number to their debit card, not having enough money in their account to pay for said transaction with the card they are using, and not being able to fucking add the total of the shit they are buying before they get to the register to be much more of a problem.

Nothing is more aggravating then having someone with 150 items in front of you at the grocery store only to have to have the cashier take off 40 of the items that were rung up so the total will get low enough for their debit card or check to go through.

The cash equivalent of this is the idiot that buys a $50 phone card, a $15 phone charger, and a $60 dollar video game while having $80 dollars of cash on him and is shocked to find out the total after tax is in excess of $125 dollars. Of course this idiot is sure that something must have been rung up twice.
This isn't advanced calculus you fucking moron. It's simple fucking addition.

The only people who may possibly be considered more annoying at a checkout line are the douchebags who have to wait until right when they check out to call someone about something completely unrelated to the transaction and wait until they are done talking before they pay. Hitler was right about these people. They should be sterilized as quickly as possible.

Stupidity-causing 99% of the world's problems since the invention of fire.