- Epileptic Trees: Given the way the Primords and the goo interact, it can raise the question of whether the goo is in fact alive in some way, and wants to spread...
- Harsher in Hindsight:
- Hilarious in Hindsight: Watching the Brigade Leader get his ass kicked by a scientist becomes even funnier when the Brigadier knocked the Master into a coma with only one punch in "The Five Doctors". Indeed, the Brigade Leader is nowhere near as skilled in combat as his counterpart.
- Moral Event Horizon: The Alternate Brigade Leader crosses this when he tries to kill the Doctor for refusing to bring him to the original world, even though doing so would destroy all of reality.
- Sacred Cow: It is widely considered by fans and critics as the best serial of the Third Doctor.
- Special Effects Failure:
- The Primords. While the victims in the early stages of the transformation actually do look quite creepy, the fully-transformed ones look as though they've gotten lost on the way to a fancy dress party.
- The otherwise wonderfully effective cliffhanger of Episode 6 is dashed by an incredibly phoney-looking greenscreen effect.
- The Woobie: Sure, they're fascists, but all the Mirror Universe people die horribly. The only exception is Director Stahlman. Even the Brigade Leader and Benton count.
- Vindicated by History: It got the lowest ratings of the season on its original broadcast. Now, many consider it the best story of the Pertwee era.
- Visual Effects of Awesome:
- The close up shots of the smoke and steam pouring out of the model drillhead during the emergencies.
- Mirror Benton's transformation scene.
- Although clearly just a colour filter, the outside scenes in the alternate universe amazingly capture a dying, burning planet.
YMMV / Doctor Who S7 E4 "Inferno"