Plot hole: Mrs. Kravitz, while standing in the Stephenses' front doorway, sees Santa Claus and assumes that it's Darrin in costume. However, the very next shot shows Santa exiting the kitchen (around the corner from the front door) and starting up the hallway toward the living room. There is no possible way Mrs. Kravitz could have seen him when she said she did.
Revealing mistake: Paul Lynde is supposedly "in" the mirror near the front door, but as he waves his arm to cast a spell, the shadow of his arm waving is visible at the far left, revealing he is actually standing off camera at a spot where his reflection is visible. Elizabeth Montgomery also briefly glances at him.