When Mrs. Brant and her friend see the picture of Frankenstein and Brant in the newspaper, you can tell it is a recent paper from 1969 when the movie was made, and not the 1800s time frame. Although the news articles are in German, you can easily see words like "American President Nixon." Looks like Hammer's props department superimposed the drawing of Frankenstein and Brant onto a real daily newspaper they bought that day.
The flowers in Frankenstein's buttonhole change both color and shape between shots when he first invites Mrs Brant into Anna's house. They start as three yellow and purple tulips, change to a purple and orange spray during his conversation with Mrs Brant in the living room, and change back as he guides her down the laboratory stairs.
When Professor Richter/the Creature pushes Karl out of the way and picks up the baron, Karl is laying unconscious on the front walk. The Creature then carries the baron into the burning house, and in the following widescreen shot, the front walk is now empty, Karl has disappeared.
A crew member is visible just after the water-pipe bursts, about one hour into the movie. As Frankenstein enters the house and walks down the hallway, the camera pans across to the left, and as it does so, someone is seen disappearing quickly behind the doorway to get out of shot.