Customers who randomally decided to all que at once and despite the fact that various staff members can probably be seen floating around, they do not think to ask one of us to open a till until the que has gotten HUGE. Alongside this - customers who que up at multiple tills when it's going to be only at one till until I can get hold of someone else to help get it down, then getting huffy at me because I served someone else before them - it is not always immediately obvious as to who was first and who has just been stupid and gone to the wrong till - waiting an extra 2 minutes isn't going to kill you. This is also made more difficult if there are so many people that they're all sort of mixed in.
And again, the whole self-entitled customers thinking they're entitled to a refund no matter what - get over your selves already, not all stores are going to be as generous as M&S when they don't have to be =/. This is going to continue to grate on my nerves no matter what, particularly because they can just get so in your face and "But I'm right! The *customer spews out some law which they have completely misunderstood* says you have to give me my money back!" is the most annoying, though the look on their face after your correct them that they are indeed, incorrect, is often completely priceless.
E: Just back reading - we also have a mix of card machines available - we get quite a few americans coming off base and sometimes we have issues with certain bank cards like RBS, NatWest etc on some tills, so we have some that have the swiper on them and some that don't. So bloody often I get people taking their card (which clearly has a chip on it - the clue is in the name - CHIP and PIN), who then insert the card vertically into the swiper bit. I get this happening all the time and then they look confused when I tell them it goes in the other bit. I get that some places like Boots require you to put it in vertically but seriously, how is this becoming such a widespread phenomenon when in my experience, those kinds are readers are quite the minority?