I never have this experience these days as I do all my shopping online and just collect it at a point far away from checkouts.
However, when I was able to shop like a normal person (have I ever been a normal person?) I always seemed to get stuck behind someone whose trolley was full to the brim with dozens and dozen of loaves of bread.
Or a person with a trolley piled high who then quibbled about many of the prices, necessitating a bell to be rung and a supervisor to check. <sigh>
,sigh.
Usually, this same person, when told the final total, only then started rummaging for her purse.
I do remember, ages ago, a man in front of me desperately trying to juggle packages which kept slipping out of his arms (he should have been using a basket) and each time he bet down to pick one up, another would fall.
I thought he was carrying chocolates but when he'd been checked through, the assistant was giggling so much I asked her why.
'Didn't you see that guy had 25 packets of condoms? He's going to have a wild weekend!'