I mostly learned English from the computer and books, and I could never understand how it is even possible to write "it's" when you mean "its", or vice versa. They are simply completely different words!
Then I went to my first multi-day conference abroad, and after three days of speaking mostly English, I caught myself doing the same mistake. I guess it's easy when you think in sounds, and not in letters.
@deshipu I think you're right, it's mostly native and more fluent writers that get this wrong and likely, as you say, in proportion to the amount they speak/listen rather than read/write.
@deshipu It's not simple, though; 200 years or more ago the spellings of those two words were generally the other way round - though spelling was less homogeneous then anyway.
It's understandable that people think “it's” is possessive given the use of apostrophe “s” on nouns for that though “his” and “hers” ought to be a clue.
“One's” is just evil.
@edavies I don't think it actually matters whether they are consistent or "logical" or not — spelling is something you learn by use anyways. Contractions are ambiguous like all other abbreviations — that's the whole idea, you make it shorter but still understandable in context. English is a low-context language, so it's not that bad.
I'm just so annoyed by those mistakes popping up in newspapers, books, articles, etc. — it just shows that the author didn't even bother with proofreading.