It's well-documented that applications must call mongoc_init once at the beginning and mongoc_cleanup once at the end. If an application is written incorrectly, interesting things can happen:

/* allocates metadata values */
mongoc_init ();
/* frees metadata values */
mongoc_cleanup ();
/* no effect */
mongoc_init ();
/* involves allocations, maybe overwrites metadata values */
client = mongoc_client_new ("mongodb://user:pass@localhost");

The first call to mongoc_init allocates the handshake metadata values such as the driver name, and the first call to mongoc_cleanup frees these values. After that, subsequent calls to mongoc_init and mongoc_cleanup do nothing, by design.

However, creating a mongoc_client_t after mongoc_cleanup uses the metadata values, even though they were freed by mongoc_cleanup. Since they're freed, they may be overwritten by some future memory allocation. It's unpredictable what data will be written there and used as the metadata.

With code like the above, I produced a garbage handshake, including a scrap of the URI, since the driver allocates space for a copy of the URI after mongoc_cleanup, and the space previously occupied by the handshake metadata is reused for the URI copy.