Another time travel show reference (Dr Who). John Constantine talking to Ava and Gary "...My business card says Master of the Dark Arts not Doctor ruddy what's his face". Ava and Gary both say "Who?", Constantine then replies "Exactly".
In this episode, Sara initiates the Nostromo protocol. This is a reference to the spaceship in Alien where one Xenomorph hunts and kills the crew.
Ava says that John Constantine is 36 years old, Matt Ryan was actually 36 years old at the time of this episode.
Sara's appearance when she's under Mallus's control also makes her greatly resemble the DC supervillians Livewire and Silver Banshee.