If you made a mistake in your NIOS class 12 exam OMR sheets such as marking the wrong set code you should not worry because examiners are trained to handle such issues during evaluation. Your roll ,Subject code and other details help them match your paper correctly so one small error does not cancel the entire sheet. If a question is left blank or marked incorrectly that particular answer may be treated as invalid but the rest of your sheet will be still be checked The best step now is to stay calm Focus on your upcoming subjects and wait for the official result which will be published on the NIOS website.