When I studied Russian there was an older student whose mastery of the language was so brilliant that native speakers assumed he was Russian. Unfortunately, his 99.9% mastery left room for 0.1% errors, and these were the kind of errors a native speaker would never make, so they sounded odd to Russians. Instead of recognizing him as a brilliant student, a lot of people took him for an illiterate (literally "grammarless") Russian and wrote him off as an idiot. A foreign accent has its advantages!
[1] In case it's not obvious, this being the internet and all, I'm complimenting you and I'm envious.