This happened to me once, but I quickly said "how are you-getting by? Seems kinda slow today." I was like ok, could go back to that store again. We're good.
One time i got popcorn at the movies and the cashier girl said "enjoy the movie" and i said "thanks, you too." I realized what i had done right away but the girl didn't seem to notice so i thought it was all good but my freaking mother was with me and she called me out on it right there in front of the cashier before i could walk away
It's the beginning of this!