On OSX 10.10 the XCode Instruments Memory Leak profiling tool reports a 16-byte memory leak for every mongoc_collection_get/destroy sequence. The tool attributes the leak to mongoc_buffer_destroy(). The sequence to reproduce is:
mongoc_init();
mongoc_client_t *client = mongoc_client_new("mongodb://localhost:27017/");
mongoc_collection_t *collection = mongoc_client_get_collection(client, "testdb", "testco");
mongoc_collection_destroy(collection);
mongoc_client_destroy(client);
mongoc_cleanup();

This seems like such a fundamental sequence, possibly the profiling tool is in error?