From http://mongoc.org/libmongoc/current/mongoc_collection_t.html:

It is an error to call mongoc_collection_destroy() on a collection that has operations pending. It is required that you release mongoc_cursor_t structures before calling mongoc_collection_destroy().

From http://mongoc.org/libmongoc/current/mongoc_database_t.html

It is an error to call mongoc_database_destroy() on a database that has operations pending. It is required that you release mongoc_cursor_t structures before calling mongoc_database_destroy.

From http://mongoc.org/libmongoc/current/mongoc_client_t.html:

It is an error to call mongoc_client_destroy on a client that has operations pending. It is required that you release mongoc_collection_t and mongoc_database_t structures before calling mongoc_client_destroy.

Per discussion on Slack, mongoc_cursor_t only cares about mongoc_client_t. Is that true for other types as well? Please audit lifecycle constraint documentation and update as needed.