I have a colleague who is a non-native speaker of English, but who speaks basically fluent English. One gotcha is that he refers to “scrap paper” as “crap paper”—which, when you think about it, isn’t too unreasonable. It’s not unreasonable that “crap paper” could be a commonly-user term for paper that doesn’t have any focused use. I’ve been procrastinating for years about whether to mention this infelicity; it is probably too late now.
Bigger lesson—it is hard, when learning a language, to hoover up that final 0.01% of erroneous knowledge.