Revealing mistake: During the Doctor's early conversation with the timelord, there is fog swirling around them. But when the Doctor agrees to take on the mission, it suddenly appears very concentrated around the tip of the rocks behind them, as if generated from a source behind the rocks. Normal fog would not do this, and it's clear that dry ice was used as the fog effect (DVD commentary confirms this).
Revealing mistake: Whenever there's a close-up of Davros, it's very easy to see that there's a gap around his mouth where the mask should blend in with the actor's face, but doesn't. This mistake was rectified, incidentally, in Davros' next appearance in Destiny of the Daleks.
Revealing mistake: In the last part, the giant robot steps on a soldier. There are several things wrong with this scene. 1) You can see the matte halo around the robot's foot. 2) The robot does not leave a footprint. And 3), after the soldier gets squashed, his arm is visible waving over the robot's foot.