I am fully bilingual and know quite a bit of a third language. Given I have learned my second language fairly late in life - around 16 - I do have some accent. But I do not mess up grammar in a way that would make your ears or eyes bleed. Or so I think ;-)<p>I have two kids that we raised with two languages since birth. And now the interesting part: 
- daughter is fully bilingual, native speakers of both languages cannot tell she is not native,
- son has some handicap in one of the languages.<p>I really think it goes the other way - having some cognitive advantage will allow you to be bilingual 100% and not the other way around. This is why my daughter can pull this stunt and my son not so much.