A common (but hard-for-humans-to-spot) typo in Hiera data is namespacing parameters with a single colon instead of a two, like:
mod_name::ns1::ns2:parameter: value
instead of:
mod_name::ns1::ns2::parameter: value
It could save the industry a lot of collective time and head-scratching if Hiera or `puppet module lookup --explain` could emit a warning when encountering any key name that contains a single colon (:) instead of a double colon (::).