Revealing mistake: In addition to Samantha, Elizabeth Montgomery also played Serena. In scenes where both characters appeared, Elizabeth would face the camera and play one character and another actress, Melody McCord, would play the other with her back to the camera. But in this scene, when Mr. Beams tells Serena she has a luncheon date with her intended at the rotisserie, Melody (portraying Samantha), with her back to the camera, inadvertently turns her head, revealing her face. (00:07:20)
Continuity mistake: The minute hand on the teakettle clock is between the 6 and the 7 when it's shown behind Samantha. When she goes to the closet, the clock reads 8:15 (minute hand on the 3). When she returns, the minute hand is between the 6 and the 7 again. (00:02:20)
Other mistake: Samantha is reading a 4-line note from Endora. The paper is semi-transparent and in a shot from the front, we can easily see the writing through the paper. However, after the note is "twitched" into a second, shorter note, there's another shot from the front - and the lettering looks exactly the same as it did before. (00:16:40)
Other mistake: When Darrin is first shrunk we see that his clothes are unaffected by Endora's spell (they don't shrink). So, when he was popped back to normal heights, his clothes shouldn't be affected, and he would burst out of them. We already know his suit changed colors.
Continuity mistake: After Samantha sees the statue talking to her, her expression goes from shocked to annoyed and back to shocked in consecutive shots. Also, her hair is over both shoulders in each of the "shocked" shots but over one shoulder in the "annoyed" shot. (00:24:30)
Revealing mistake: At the end of the episode, during Serena and Samantha's second meeting with Mr. Beams, in the wideshots of the two women sitting on the couch and also when they're both poking their heads through the doorway, Serena's body double actually appears face-forward. Additionally, in the closeups of Serena and Sam on the couch, both are played by Elizabeth Montgomery, and we can see the wall's color difference behind them due to the use of the split-screen technique. (00:22:25)