Susan is wearing knee socks in the reprise from the previous week's episode, but in the following scene she is not wearing them.
When Ian checks the Doctor he says his heart is alright. Like all Time Lords the Doctor has two hearts.
Shadows of two crew members are visible when Susan enters the control room.
Early in the episode as Barbara tends to the doctor, a shadow is seen moving behind Susan. When Susan leaves the scene, the shadow remains.
When the Tardis crew go to look at the scanner after the Doctor turn it on, the boom mic and camera are reflected in the television screen.
There is a shadow visible on wall when Barbara tries to wake Ian up.