I was once the person who thoroughly annoyed the person behind me at the till, and I was only in my early twenties at the time.
My one year old was sitting in the trolley seat.
The customer in front of me was still being served, so in order to get my goods onto the counter as space became available on it, I had to put the trolley behind me instead of in front of me.
I managed to get everything on the belt just as the assistant was ready to begin checking my things through so, trolley still behind me, I started packing straight away.
As my last item went went through, I turned for my trolley, but it had gone - the impatient b*****d behind me had moved it and my baby away, out of my sight between our till and the next. The man didn't give me time to say anything before he started having a go at me for apparently holding him up by leaving it in his way so that he "couldn't get served".
There were several things about that incident that really upset me. Firstly, he could have easily got his few items on the counter without having to abduct my child. Secondly, up until that point I had done everything possible to get through the transaction as quickly as possible, and as I hadn't even had time to pay yet (because of him!), it wasn't even his 'turn', yet as he was claiming. Thirdly, not one person in that queue, not even the cashier, said a word. With hindsight, I should have kicked up a big fuss, accused him of child abduction and refused to pay until the matter was resolved in some way - in other words, I should have really made him have to wait to be served, but I wouldn't say boo to a goose back the.
I left that shop in tears, and how he made me feel has stayed with me.
And that is one reason why I will never be nasty enough to say something at checkout that might cause someone else feel like that just for being a bit slower or less organized than I would like.