Deliberate mistake: Look for the "creases" in the wall inside the TARDIS, which look as if the wall was just a sheet hung up behind the console. (In fact it actually WAS a sheet and not a "solid" wall - the TARDIS wall prop was apparently unusable due to water damage, due to poor storage between seasons. This also explains the lack of TARDIS interior scenes throughout the rest of season 26: it was not thought worth repairing the TARDIS set [or building a new one] when the show was in imminent danger of being cancelled after season 26).
Deliberate mistake: One major mishap occurred just prior to editing, when a videotape containing various close-up and insert shots was accidentally wiped. At that point, there was no way that the director (Nicholas Mallett) could go back and rerecord the material, and so he was forced to work without it. This meant using footage that would usually have been discarded or replaced. The 'give aways' that reveal that Mallet had to work around the problems of the missing footage are (a) the lack of any close-ups of any of the actors in the final 10 minutes of episode 4, and (b) the continuity error involving the flames in the laboratory for the same period...the flames go from low to high and back to low from shot to shot.
Deliberate mistake: In episode three, the Doctor and Vira meet a half-changed Noah. He holds a gun on them, then drops it at Vira's feet and the door is suddenly closed. This is because the scene was originally longer and involved Noah begging Vira to kill him before the Doctor shuts the door. It was trimmed down to get it past BBC censors.
Deliberate mistake: In episode 3 of 'The Web Planet' (titled 'Escape To Danger') watch as one of the Zarbi operators, his vision impaired by the costume, runs right into the camera. So far behind schedule was the recording of this episode, however, that this mistake was retained in the finished episode, as there was no time left for a re-take.