This was maybe two months ago, pretty mundane situation but it still gets me when I think about it.
I'm at the self checkout at my regular grocery store, the kind with like eight machines and one attendant floating around. I had a pretty full basket, maybe 25 items, and I was about halfway through scanning when I stepped back to grab my bag from the little shelf behind me. Took maybe four seconds.
When I turned back around there was a woman standing at my machine scanning her own items. She had just walked up and started using it while my half-scanned order was still on the screen. I said excuse me, I think I was using this one. She looked at me, looked at the screen, and said "well you walked away." I said I stepped back for literally one second to get my bag. She shrugged and kept scanning.
I stood there for a moment genuinely not sure what to do. Then I flagged down the attendant and explained what happened. The attendant came over, saw my half-scanned order on one side of the screen and this woman's items being added to it, and had to void the whole thing and separate the transactions. It took about four minutes.
The woman huffed the entire time like we were inconveniencing her. At one point she said she "didn't have all day." The attendant was very polite about it which honestly was more than I could have managed. She left without saying anything. I finished scanning my actual groceries.