As The Doctor, Benton, and the Brigadier leave the TARDIS, they leave both its front doors wide open, revealing the unexciting reality of the actual prop interior.
Boom mike lowers into frame when the Doctor says 'You knew your mission was dangerous' to Omega.
At c.1:22 in as Doctor 3, Jo and Tyler enter the chamber for their first meeting with Omega, he appears shadowy in the archway at the back of the set whilst Jo is looking around in wonderment(?) at the place, she looks directly at Omega and then turns quickly away as if she hadn't seen anything. Perhaps Stephen Thorne was a little early on his cue or Katy was being over zealous and took too long in her study of the chamber.