The YAML frontmatter (note: Frontmatter) cannot be parsed (7 passages, no note). The body is still processed; every declared attribute (note: Attribute) is lost.
Check (note: Check) E-FM-INVALID, severity (note: Severity) error (29 passages, no note) by default, documented with a before and after example in the check pages. The severity can be overridden per project (note: Project) or per repository (note: Knowledge repository).