Yeah I've heard/seen that a lot. I know a few foreigners with great English, but they still make mistakes all the time. It must be one of the least consistent languages out there. But most languages probably have some interesting surprises at the "advanced" level. I learned French for a long time. It's hard at first because the spelling/pronunciation is weird for an English speaker. Then it gets much easier, because French is in fact a fairly consistent language with nice structure. However at the advanced level French has quite a few surprises. Strange inconsistencies preserved for historical or aesthetic reasons, opaque idioms, surprising new tenses, chains of relative clauses to make your head spin...