There are stacked boxes in the dungeon with Cyrillic (Russian) lettering. However, the lettering is mirror image as if someone who didn't know Russian painted on the stencil reversed. The film itself is not reversed since another sign in Englush is shown correctly.
The double for Diana Rigg is a blonde.
At the dinner table only Steed's brandy glass was vibrating.
(at around 19 mins) just after Steed puts the paper boat into the moat. A wire can be seen pulling the paper boat.
About 6 minutes in, "Angus" takes Mrs Peel to the banquet room. Twice in this brief scene you can see the boom mic drop into the frame right over Mrs Peel's head.
When Angus and Steed are sword fighting, near the end of the story) it's clearly a double for Angus in the distance shots.