To provide a more detailed answer:
Can an application be returned because you missed writing N/A on the checklist? Short answer is yes. It can and has happened before. Long answer. Will that happen to you? Not necessarily. The officer who picks up your file may just let it slide. There is no guarantee either way. At this point now that it’s been sent, I would suggest doing your best not to stress about it. Hopefully it won’t be an issue.
Now onto the reason why they would return an application for that. As I said in a previous post on another thread. The reason you are asked to write N/A rather than leaving it blank is because by writing N/A you are confirming that you have seen, understood, and acknowledged the instructions and what is being asked of you, and you are confirming that those sections of the application do not apply to you. That way they know you have not missed or purposely left something out of the application.
It is the same reason why throughout your application if at any point you cannot provide what they are asking for or don’t know the answer to something you are asked, you have to add a separate sheet of paper with an explanation. That way you are confirming that you have read, understood, and acknowledged what was asked of you and you have answered by providing an explanation of why you cannot provide what is being asked of you. Again, that way they know you are not missing anything accidentally or purposely withholding information.
Doesn’t matter if it’s because you don’t know a family member’s address, father’s name is missing on a birth certificate, can’t provide documents or whatever. An explanation needs to be given for any missing information you are unable to provide for one reason or another.