Sometimes it happens by accident. I've caught myself typing incorrectly before, and it's not like I didn't know the difference between "your" and "you're" since I learned how to read English.
Haruna and a lot of Japanese people aren't that proficient in English, plus she's not required to be for what she does so it's fine, but ...it's just that I'd personally look up and double check on that kind of stuff before I put it on an artwork to show to the public. Even with just Facebook statuses, I'd often go back to check a few minutes later to make sure I didn't type anything incorrectly.