To add to your examples, there's also the case that you've already unloaded your cart, but the idiot in front of you sends the cashier on a wild goose chase to get cigarettes for him and they spend ten minutes sorting that out. Or you're behind some "extreme couponer" who splits their order into six separate orders to maximize their coupon savings, and you have to resist the urge to just pay for their entire order just to shame them into being less of a goddamn miser.
And then you CAN'T leave the slow line because there are people boxing you in from behind, even though you chose this line because the guy ahead of you had only one item... until he decided to make the cashier search the entire store to make sure they were actually out of his favorite cigarettes.
