When Emma Peel consults the church archives she wears gloves and when she points out something written on the page to the vicar, she is bare handed.
Just before going to the church, Mrs. Peel's nails are not painted. However, in the church, when there is a close up of her hand on a church archive, her nails are painted. (someone else already pointed out another goof in this same scene where first she's wearing gloves and then she isn't).
When Steed goes to Mrs. Peel's, he uses his umbrella to ring the doorbell. Although the doorbell button doesn't move, the bell still rings.
When Smallwood visits the blacksmith, he places his suitcase on the floor, and leaves it there when he leaves. When he subsequently approaches the church, the frontal shot shows both hands are now in his pockets. When the camera angle shifts to his rear, the suitcase is back in his right hand.
During the scene in the underground bunker, the film has been reversed. The lettering on the door control switch box is reversed.
During the scene in which Smallwood is chased by bloodhounds, there is a shot of him running through some water - but whereas the actor playing Smallwood is obese and dark-haired, the stunt man seen front-on in the water shot is grey-haired and slim.
When Mrs. Peel is first seen on the sand dunes looking for Steed you can see part of the equipment holding the painted background in place at the top right.
The shadow of the boom microphone is visible in the upper left when Mrs. Peel is showing the record book with the missing pages to the vicar.
When the vicar says, "It's a very appropriate piece, Mrs. Peel. It's a Requiem", the music playing is not from a Requiem. It's Mozart's "Ave Verum Corpus".