Murphy's Law of Checkout-Lines
"If you aren't in a hurry to leave the store, there will be little or no delay in getting a clerk to ring you up. But if you're pressed for time --- or even more so, if you're car-pooling with someone who is on a tight schedule --- THAT'S when there will be hold-ups galore... a long line in front of you, a customer with one or more heaped-full shopping-carts, one or more price-discrepancies that hafta be tediously resolved (i.e., the puzzled clerk is obliged to walk back with you to the shelf to look at the price-labels, a supervisor or the store-manager has to be called over), a balky card-scanner, etc."
Whenever someone who's giving me a ride is kind enough to let me stop at a store to grab a few items, I always try to make very sure that I have everything all "set and prepared" before I head to the register, so that I personally seldom create a problem (i.e., cause any delays myself with issues like not having the proper payment ready or being unsure of a price or quantity) with Murphy's Law of Checkout-Lines. That way, even if my driver has to wait linger than expected for me to hastily scamper back outside with my loaded shopping-bags, he won't be able to direct his impatient frustration at me --- "Sorry for the delay; I did indeed get everything tossed in my cart very quickly, but there was a long line in front of me, and they were short-staffed today, so they only had once register open."