The End in the Beginning - S4-E26
Plot hole: We learn that Agent Booth has a brain tumor that has been causing him to hallucinate. One of the hallucinations mentioned was Parker in "The Hero in the Hold." At the end of the episode, Dr. Brennan both sees and speaks to Parker in the cemetery. Agent Booth believed Parker to be a ghost, and we were to believe that Dr. Brennan had unknowingly had a paranormal experience.