큰문제 있죠? 영어 가르치는 분 잘 못했어...

On Twitter today, I saw this:

오늘영어한문장 - How about going for a coffee? (커피마시러 가는게 어때요?)

Well, actually, this isn't quite right.  The problem is that if you learn this pattern, you will MAKE MISTAKES IN THE FUTURE.

(x)  "How about going for a coffee?"
(o)  "How about going for a cup of coffee?"
(o)  "How about going for some coffee?"

The problem is that coffee is a noun which is actually plural, not singular.  As a result, the article "a" is wrong.

Now, the issue is that the teacher in this case is not a native speaker of English, and posting on Twitter as if she is completely fluent.  This is not the case.  In Korea, as you probably know, there are many so called "experts" who are not actually experts. 
It is the right of students to ask for a teacher's qualifications to teach, and to choose teachers that can actually be considered as expert.