My friend used to work in a bagel shop. One of the owners was Italian, and her nephew came to work and to improve his English. I'll never forget a customer coming up to him and saying 'You speak English so good!' All of the facepalms. All of them. I waited until she walked away and I corrected the sentence for him. I think his favorite thing was all the colloquialisms and such: trash basket/bucket, garbage pail or can, trash can, waste basket. 8 different ways to describe a container in which you throw garbage!
At my job, I get the WORST math mistakes ever, though. I think that hurts me more- the two numbers you're trying to add together are RIGHT ON TOP OF EACH OTHER. How hard is this?!? Or when gratuity is added and suddenly math/logic becomes a completely foreign concept. "But I don't understand where this number is coming from?" 'Well, when you add the cost of the meal, plus tax, plus 18% of your pre-tax total, you get XYZ.' "But, how do you figure it out. And there were 6 of us at this table but only 4 of us ate, does that matter?" Are you REALLY that cheap? Yes it does matter- you took up a table from a paying customer. If you can't function in a public sit down restaurant and don't know how/want to tip- there is a McDonalds down the street.