As of r1612418, both the 4x and trunk svn trees seem to have a bug where any core specific init errors that occur on startup don't show up in the log until/unless someone attempts to access that core via HTTP.
i'm not sure when exactly this bug was introduced, but it definitely isn't in 4.9.
The impact on users, particularly new users, is that starting up solr with a mistake in your configs appears to work fine until you actually try to use solr and then you get ugly errors.
- is part of
SOLR-6232 Allow cores that have failed to init to be deleted via CoreAdminHandler