in function: mongoc_collection_find_and_modify
if reply is created by bson_new(), will cause memory leakage.

code:
use example in
http://api.mongodb.org/c/current/mongoc_collection_find_and_modify.html
just replace

  bson_t reply;

with

  bson_t* reply = bson_new();

then will cause memory leakage:

  bson_t* reply = bson_new()
...
  if (!mongoc_collection_find_and_modify (collection, query, NULL, update, NULL, false, false, true, reply, &error)) 
...
  bson_destroy (reply);
...

reason:
mongoc_collection_find_and_modify use bson_copy_to() or bson_init() to set reply. These two function used with bson_t object created by bson_new() will cause memory leakage.

eg:
1.

bson_t * doc = bson_new();
bson_init(doc);
bson_destroy(doc);

2.

bson_t * src= bson_new();
bson_t * dst= bson_new();
bson_copy_to(src, dst);
bson_destroy(src);
bson_destroy(dst);

both code piece of code will cause memory leakage.