Diane, you can be TOO honest, you know.
My pet peeve is those people who blatantly stand in the 15 Items Or Less queue when they obviously have a trolley overflowing with goods - no-one from the store ever calls them on it.
I was in a hurry once and only had two muesli bars to pay for. I got involved in one of those embarrassing foot races where two people eye up the same checkout and increase their stride to get ahead of each other, whilst trying to appear nonchalant. My competitor had a trolley overflowing with groceries, and got in first. The checkout operator, whilst clearly seeing that I only had two piddly items, nevertheless processed the other customer.
Another peeve is the chatty customer who often knows the checkout operator personally, and who carries on the conversation well after they have been served, oblivious to the fact that there are people behind them who want to get their goods put through and get on with their lives. The checkout operator is often embarrassed by this too, but doesn't have the professional, or people skills to cap the conversation.