Database inconsistencies occur when, for example, primary keys stored in other fields no longer point to a row that exists.
This can happen after a database import ends prematurely, for example.
In that case, page.page_latest might point to a revision that hasn't been imported, and users trying to read the page might get an error message stating "The revision #0 of the page named '[page name]" does not exist.
This is usually caused by following an outdated history link to a page that has been deleted.
Details can be found in the deletion log.
findAnomalies.php can be used to find missing revisions.