FilteringClientCursor::FilteringClientCursor( auto_ptr<DBClientCursor> cursor , const BSONObj filter )
: _matcher( filter ) , _cursor( cursor ) , _pcmData( NULL ), _done( cursor.get() == 0 ) {
}

The order of calling copy constructor for _cursor and evaluation of the expression ( cursor.get() == 0 ) above is not guaranteed. The problem is that copy constructor for auto_ptr resets the original.