Don't get me started...
For some reason, most normal people can't express their preferences - and they absolutely have them, which they'll be very happy to shout at you about when you come back with the wrong product. I've learned the hard way that when someone asks me to buy e.g. milk, I need to ask about brand, size, fat percentage, UHT or no, and which other brands/sizes/percentages are acceptable if the preferred one is not available - and I need to ask those questions before leaving.
Or maybe it's just me who wasn't born with mind-reading capability...
(And no, buying the "usual one" is not a good strategy, because there are good chances that they need a different one just this once, and yes, I was supposed to guess that myself.)