I have recently got a job in a local supermarket. I've been out of work for a while.
It's only my 3rd week in. Just got home and feel like a complete twat.
I had already opened the till to give change and he wanted to give me some money so it made a 5 note. I completely forgot that the amount shown on till is his change and I mistakenly thought it was the total of the shopping (I knew it wasn't but it was just a mistake) and he even said "you've given me too much change" I then went and tried to explain why I hadn't and he just went okay and left and probably thought the stupid bitch and rubbed his hands together.
I have an a level in maths as well 
The till wasn't short when counted so don't think anyone knows but I just feel so stupid.