native language interference
Teachers - in a perfect world, I'm sure you would all love to help each and every student to become as good as they can possibly get in English. To help them in this goal, you might want to correct (at some point) each and every type of mistake that they make.
But this isn't a perfect world and time is not infinite.
That being the case:
If you have a student who is preparing for a test and they have, let's say, three different types of problem with their use of English and one of them is obviously down to L1 interference and the other two appear to be unrelated to the mechanisms of their native language, would you ever consider working to eliminate the two "other problems," and leaving the L1 issue to one side on the basis that they may well be more easily correctable (given time constraints)?