Revealing mistake: Bethany gets upset about finding out she's related to Jesus and goes in the water to scream at God. Metatron is instantly there. He walks across the water. You can see the supports underwater that he is standing on. Look to the left of him.
Visible crew/equipment: When Loki kills the guy on the bus committing adultery, the camera pushes in on him before he pulls the trigger. As it is doing this, the shadow of the camera is visible on the left side. It's clear enough; you can see the clear shadow of the camera hood.
Answer: In a scene shortly before, Azrael was telling them about the last scion being sent after them. After that scene cut, he could have mentioned that she had an Apostle with her. That, and, Bartleby was a 'Watcher' Angel (a Gregoriate if I remember correctly), and he can see into people's pasts.
MoonFaery ★