common mistakes

I teach English and I sometimes wonder how grave certain mistakes made by my pupils are.  For instance, congratulations for instead of on.  Is it considered a grave mistake by natives?  If so, how grave is it.  Second question.  Being tea an uncountable noun I´m supposed to order two cups of tea but could I say two teas, please?
Thanks for your answer
Carla from Argentina

HI there. English is not my native but I am married to a native, and English has become our family native language. Language which is evolved from a custom will be different from a textbook based knowledge. I have not heard an Australian saying, two teas. But they do say, "The food is lovely." as opposed to Bristish saying, "The food tastes fanstastic and it was delicious." But that is an everyday thing. On the books, however, it will be proper to teach the students according to the requirements set by their language programme so as not to confuse them.