In October 2021, South Korea said it had detected a ballistic missile fired from a North Korean submarine. Japan initially said it was two missiles.  

The U.S. and its Asian allies have for years collected lots of information on North Korean weapons tests, but the data has still offered only a partial and sometimes imperfect picture of what has been happening in real time. In the 2021 test, it turned out to be just one missile—Japan later corrected its initial statement.