Well, I do not consider myself to be a native speaker of English, but (not so) recently it has become the dominant language of use in my daily interactions. In addition, I've been studying in English since the age of 15, so I would say that my level of English is comparable to that of a native speaker, if not better in some aspects.
There is one particular grammatical error that seems to get under my skin. More often than not it is native speakers who commit such grammatical blasphemy. Believe it or not, many of them do not know the difference between "there," "their" and "they're." Though all of these words are homophones, their meanings differ greatly. So, Russians (and native English speakers), take note:
There: a demonstrative pronoun, often indicating location. (Here vs. There)
Their: a third personal plural possessive pronoun. (My, your, his, her, their)
They're: They + are. Enough said.
A good example that utilizes all the aforementioned is this sentence:
"They're fixing their car over there".
Hope that helped