(at around 28 mins) When the wooden gate is sprayed with the wood rot. The gate disappears and so do the chain links that locked the gate shut, seconds later the chain reappears on the ground where Tara King picks it up.
At about 42 minutes Tara used acid to erode away the chain in order to escape. The shackle was still on her ankle at the time. But when she is in the other room seconds later the shackle is gone.
After Steed spoke to Pym, George and Kenneth are seen talking in the church gate. There is strong sunshine evidenced by shadows on the actors and on the church. When the camera angle changes, the light is different, no shadows and a cloudy sky, suggesting that the different camera angle was filmed at a different time of day.
When Mrs Forsyth brings in the tea trolley, both George and Kenneth are hatless, in fact Kenneth uses his hat to conceal his gun.
When Steed starts the fight however, both Rotters are suddenly wearing their hats.
The floor of the belfry where Pym is working is clearly wooden floor boards. Therefore, when the Rotters sprayed it with dry rot it should have been destroyed along with the rest of the bell tower.
The five students in the photograph of the Institute of Timber Technology are not the same people as the characters Pendred, Pym, Sawbow, Wainwright and Forsythe. They also appear too old to be prewar college students.
Whilst at the church, George remarks that there is a hint of autumn in the leaves. The scenes with Tara in the woods however show bracken growth meaning that filming actually took place in late spring or early summer.
In the credits, the actress who played Sonia is credited as Tony Gilpin rather than Toni Gilpin.
During the fight at teatime, the face of Steed's stunt double is clearly visible more than once.
The beam of the filming lights is clearly visible on trees during Tara's fight scene in the forest.
Wainwright stated that the dry rot affects the wood to which it is applied and the surrounding area (which explains why the pencil disintegrated) however when The Rotters destroyed Sir James Pendred's doors, the timber door surrounds were unaffected.
When Sandford stops Tara by moving his truck on the crossroads, she gets out of the car and runs away from him. However, as there is enough space left, this could have been entirely avoided if she had simply drove on, around the truck.
When Steed returns to his apartment after his first meeting with Pym, Tara comments that two people in the photograph have already been murdered. This wasn't true; only one person in the photograph had been murdered: James Pendred. Professor Palmer had also been murdered but he wasn't in the photograph.