A YAML parse error in config.yml (e.g. a stray tab character) caused
read_config to swallow the exception and return {}, disabling categories
and producing a broken nav.yml with no variants fields and wrong filenames.
read_config now raises ClickException on both OSError and YAMLError.
Documented in docs/knownbugs.md and docs/unreleased.md.
https://claude.ai/code/session_01Xs5GyREFhjWxhS1UhW2wA8
