This is a maintenance category of articles with invalid ISMNs. It is often included by means of the templates {{ISMN}} and {{Please check ISMN}}, which should be located next to the ISMN with the problem.
Articles can also be included in this category if an ISMN within the {{ISMN}} template is not valid.
The most likely reason is a bad checksum, (see ISMN) which probably means there is a typing error in the ISMN.
See also Category:CS1 errors: ISMN