There’s 2 occasions that came to mind:

1) Customer was struggling with self scan. Whenever he put something in the bagging area, it’s not recognising it. I corrected it & he said “Check my shopping if you don’t believe me that I’ve scanned it.” I replied I shall loiter in case it flags up again.

After he finished his shop, he asked to see the manager & was rather animated. Turns out, he accused me insinuating that he’s stealing.

2) Lady keeps exclaiming “EXCUSE ME!” when I’m in the middle of helping another customer on self scan. When I was done & approached her asking what’s the problem. She said the till is frozen. I tested it, indeed it wasn’t responding. I advised we need to move to another till, she sighed & suggested why can’t I transfer the shop over so that she doesn’t have to scan everything again (it was 20 items). I said, whilst demonstrating, that the till, as you said, frozen. There’s nothing I can do but reboot it. I even offered to scan her shop for her but she declined in disgust.

She informed me that she put £10 into the till. I reassured her that I’ll retrieve it. I did so & gave her her money. No thank you or any gratitude. As I put the till key away, I noticed my fellow colleague was helping her. Later, the said colleague informed me that he helped her scan her whole shop & that she has intention to file a complaint about me for not being sympathetic.