In _finalize in mongocrypt-ctx-encrypt.c, the error case starting on line 2011 (in commit 2ec9c3) appears to be missing bson_destroy (&converted). This is because in the case where ectx->explicit is false on line 1985 and the bson_t converted is initialized on line 1996, if the call to _mongocrypt_buffer_to_bson fails on line 2009, _finalize returns without having destroyed the converted instance.