BSONObject, BasicBSONObject, etc. claim to return the value passed to it, but they appear to rely on java.util.HashMap which returns the previous value: http://docs.oracle.com/javase/7/docs/api/java/util/HashMap.html#put(K,%20V)

example of JavaDoc of BSONObject, BasicBSONObject, ...:
{{
@return <tt>v</tt>
@return the <code>val</code> parameter
}}

P.s.: I was actually hoping it would return the BSONObject itself so I could chain put() operations together