This is related to the existing bug QTBUG-31468:

Per http://dev.mysql.com/doc/refman/5.7/en/mysql-library-init.html

You must either call mysql_library_init() prior to spawning any threads, or else use a mutex to protect the call, whether you invoke mysql_library_init() or indirectly through mysql_init(). Do this prior to any other client library call.

So in addition to not being thread-safe per QTBUG-31468, if an application calls addDatabase() for the first time on a thread, the init() can fail in a quite inscrutable way - see http://stackoverflow.com/q/38343945/1296734

Until the plugin can be fixed, can the docs be updated to warn about this gotcha?