When young William Wilson splatters his fellow schoolmate with a tomato in class, the red stain on his victims face changes drastically from less to more after several moments.
In the third segment - Toby Dammit, at the end when Toby put the Ferrari in reverse and backed it up in order to get far enough back to enable him to jump the gap in the road ahead, he's wearing a white ruffled shirt open at the neck. But in one of the brief shots showing him backing up the car, he's wearing the same black jacket over the white shirt and the black ascot covering his neck that he wore earlier at the awards ceremony.
The woman Wilson ties to the dissection table is implied to be completely nude throughout the scene (and is shown as such in overhead shots), but she can be seen wearing panties in one tracking shot (when Wilson claims to the other students that he will be freeing her "from the source of all her troubles").
When Countess Frederique ("Metzengerstein") is urging the black stallion into the sea, her ermine-trimmed cloak is suddenly gone.
Toby is offered a magazine pictorial in which he is to portray "the young Greek god Mars" (as translated in captions). Mars was the Roman god of war. The Greek god of war was Ares.
During the final card game between Wilson (Alain Delon) and Giuseppina (Brigitte Bardot), Wilson deals himself two cards in a row during the final hand. He should have dealt Giuseppina a fifth card first before taking his.