[B]Hi ladymacbeth.
Just because you did'nt see something quickly enough does not mean you are stupid, i worked in a busy bar and many busy cafes and in very busy kitchens.Making a mistake does not mean you are daft or slow or stupid.
If you need the job and are not comfortable behind the till, ask you're boss to be doing something else.
I had a case in a bar once where someone claimed they had paid for their drinks when they had not, it made me angry inside and it triggerd me into thinking it was my fault. Customers sometimes try things on to see if they can get away with it, its normally the one giving the fake bill who the most nervious because they are scared of getting caught (a bit like a shoplifter/thief). Dont let it trigger you, rise above it and see it for what it is, just another customer trying to get away by giving a fake bill (belive me, its the customer who is the most nervious). Dont be afraid to do you're job if you are behind a till.
People will try anything sometimes to get away with not paying for their goods properly.
I hope this helps.

I was once a waiter too, that came with the same stress with people not wanting to pay for their food.

Take Care.[B]