Corrected entry: As the Doctor and Peri prepare to go out onto the asteroid surface for the second time, the Doctor again heads for the door without operating the control. Thankfully, Peri says something else and the Doctor turns back for a few more lines, before finally going back to the console and hitting the real door control.
Corrected entry: Azmael keeps a slug-killing potion hanging around, but never thinks of using it. With the plot of this story involving killer slugs (OK, slug-like giant gastropods), wouldn't it make sense to use anything handy that could kill "slugs" before they kill you?
Correction: Killing Mestor was impossible, as he shielded himself from the Doctor's initial attempt (only being killed on the second go as he was controlling Azmael at the time), and would presumably have done the same had Azmael attacked him. And killing the other Gastropods would have achieved nothing, as Mestor would have either killed him or had the Jocondans kill him.
Correction: The Doctor is still mentally unstable so may have simply forgotten to open the door.