My social anxiety kicked up the other day while paying for groceries and I failed at doing basic math in my hand due to my nerves. I ended up giving the wrong amount of money.
It's okay. People are very understanding. Did you underpay? I'm sure it happens all the time! 
Yup, I did! And I'm usually very good at math. When my anxiety acts up, it's like my brain just goes blank.
We all have those moments! Sometimes you just have to shake it out of your mind and take a deep breath.