Some people have bad eyesight. Some people have been deeply trained by the modern web to ignore most of what’s on the page (most of it being ads or other bullshit). Some people make mistakes.
Have some patience and kindness.
And to those who wont be patient or kind, just know that the next time the self-checkout machine yells at you and the cashier has to come scan their badge and gives you grief, you deserve it. Can’t you just use the machine correctly!?!