There's a comment at the top of BSONObj about thread safety that seems out of date.

       Note that BSONObj's have a smart pointer capability built in -- so you can
       pass them around by value.  The reference counts used to implement this
       do not use locking, so copying and destroying BSONObj's are not thread-safe
       operations.

It looks like references are counted via AtomicUInt so the counting seems thread safe. And it looks like the thread safety rules are similar to those for shared_ptr, but it would be good to check that and document.