<?php

  // Replace with the actual trashed object id
  $contentObjectId = 100;
  // Fetch trash node
  $trashNode = eZContentObjectTrashNode::fetchByContentObjectID( $contentObjectId );

  // First call : works as intended, returns trash node original parent
  $originalParent = $trashNode->attribute( 'original_parent' );
  if ( $originalParent instanceof eZContentObjectTreeNode )
    echo 'First call : success';

  // Subsequent call : fails, returns null
  $originalParent = $trashNode->attribute( 'original_parent' );
  if ( $originalParent === null )
    echo 'Second call : failure';