Sure.  So, if path is `nil` the code assumes that it is at the top level.  So, the "silently bad" message would be in the data structure `{"broker":{"name":"fred"}}`, the error message "name must refer to the name of an existing broker" rather than "broker.name must refer to the name of an existing broker".

There is absolutely no way, in the code, to detect that this is incorrect, and that we are actually dealing with a nested data structure -- which in this case would be because the path was incorrectly missed when calling validate! on a sub-schema, and so the default took over.

That is: having the default significantly increases the risk of developer error causing bad error reporting, but brings the benefit that this one line (and this one line only) does not have to explicitly pass in `nil` as the path.

To me the reduction in developer errors seems well worth the cost.