When Verdoux is at the sidewalk cafe, the items on the table change positions between shots - the white match holder is one one side, then the other, and the metal cup is on the plate, then off.
When Verdoux attempts to poison Annabella Bonheur with the "hydrogen peroxide" wine and the glasses are switched, he drinks nearly the entire glass of doctored wine. After running upstairs, drinking milk, and then being helped back downstairs by Annabella, the wine glass is nearly half full.
When Henri is talking with Maurice about the humane way to do away with dumb animals, he states that Maurice experimented with "exnyde, ethna bromide, and something else." The "something else" is then stated to be C2HC. While the first two words sound vaguely chemistry-related even though they don't exist, C2HC is a chemical formula specifically indicating three total carbon atoms and one atom of hydrogen per molecule. However, it is a fundamentally flawed formula, as carbon is a tetravalent element, requiring each atom to have 4 ionic bonds, but hydrogen is monovalent, having a single ionic bond. There is no way those 4 atoms can combine to form a compound.
In the greenhouse at the garden party, Chaplin mispronounces the name of the flower as camp-a-NEW-la. It is pronounced cam-PAN-yu-la.
When Henri Verdoux is with Marie Grosnay and falls off the couch without spilling his coffee, it can be seen that the "coffee" in the cup is not a liquid, as the level does not shift when the cup is tilted.
(at around 20 mins) When the camera moves forward to focus on Monsieur Verdoux at the café, a female passer-by is looking directly at the camera.
When Monsieur Verdoux states the area of the house being sold he mentions the lengths in feet. As the film is set in France, he should have used meters.
When Monsieur Verdoux falls out of the window, he falls on tiles yet there was no sound of him hitting the tiles.
Although the story takes place in the years 1932-1937, all the women's fashions and hairstyles are of the 1946-1947 styles, when the film was made.