Even as a native we make mistakes in our own language. Some words are just meant to give us a bit of trouble. In Danish and English for instance the words “lie/lay” – “ligge/lægge” are know to be common mistakes among even native speakers.
In Denmark it seems to be mainly one part of the country, namely West Jutland, that are having problems setting those two words apart. It seems to me that it has become such a big part of the daily language that it sounds natural to them, whereas it immediately starts an alarm bell in everybody else’s ears.
Grammar: Native vs. foreign speakers
Another thing that has come to my attention is that if you ask a native about some grammar rules, they, unless they have actually studied languages, can’t explain why it is so! As a language student you need to study and learn a lot of rules in order to master the foreign language more or less fluently.
With exceptions of cause, some people have a flair for learning other languages and seem to, by living in a country for some time, to master it just as well.
But my point being, foreigners often know your native language grammar better than you – because they had to learn it in order to master or understand your language. Inevitably the foreigners are likely to make some mistakes, especially when it comes to idioms, but they will still be able to explain certain rules or reasons that you will not!