And Those We Left Behind - S4-E6
Plot hole: The very ending scene where Peter (in this version died in the ice as a boy so Walter's wife never met him) takes off some of the dust covers then picks up a toy airplane. The toy airplane he got as a child from ... Walter's wife when she was trying to placate him in an earlier episode for trying to run "home". This toy airplane wouldn't have been his as a child in this version of the universe. (00:42:00)
Olivia Dunham: I just want to go back to before.
Phillip Broyles: Dunham, I don't think you can.
The Arrival - S1-E4
Trivia: Michael Cerveris makes his first significant appearance in this episode as The Observer/Bald Man. However if you watch the series from the beginning, he makes fleeting appearances in every episode, sometimes on a tv screen.
Question: Why would Luke Dempsey be arrested along with his father? He had no idea what his father was doing, never aided him in any way and only found out the truth after catching him in the act.
Answer: He helped his father evade capture, impeding the investigation.