Worse is taking the whole extended family shopping, having one trolley with the shopping but them all standing in the queue at the checkout belt.
Then there is the person after loading the belt and reached the till has found that they have forgotten the milk and sends their partner off to "quickly" fetch it. 10 minutes later they return with the milk and a dozen more items. I have seen in a Lidl branch where the till operator has waited too long telling the customer to pay for what has already been rung up and the missing partner returning with extra goods to join the end of the queue.
Possibly those with financial problems, but people who only judge what they can afford at the till by asking the till operator to cancel and return items once rung up. It wouldn't be too bad if it was the odd item but it does often seem to be people with £1.50 in their pocket to spend turn up at the checkout with £100 worth of goods in a trolley, including luxury items.
When you have a trolley full of goods NEVER let the person behind you with only one item go in front. They will always have one of the few items in the store that doesn't have a readable bar code. A second member of staff has to be found to get another of the same item with a readable code only to find that the customer has picked the last one in the store.