Doctor Who: 15 Cool Details Peter Harness Just Revealed About The Zygon Invasion
6. Clara's Dream Checks Are A Reference To Last Christmas
The second half of this story, The Zygon Inversion, begins with Clara waking up in her apartment. However, this isn't her normal reality: she's been taken over by a Zygon, and thus, is trapped in some sort of subconscious dream state.
Fortunately though, Clara has been trapped in a dream state before, so she knows exactly what to do in this situation.
In 2014's Last Christmas, Clara and the Doctor are attacked by Dream Crabs, which cause their targets to dream. At one point in this episode, the characters deduce that they're asleep by noticing that random words in their base manuals actually spell out a coherent sentence, just one of the few "dream checks" we see throughout the story.
In The Zygon Inversion, Clara utilises a similar dream check to help her at the start of the episode. She begins to notice oddities around her apartment, such as the toothpaste being labelled "this is toothpaste", and her alarm clock being wrong. So, she checks a nearby newspaper, to see if the words make any sense for a normal paper. They don't, of course, informing her that she's dreaming.
According to Harness, this moment was scripted in as a direct nod to Last Christmas.