The Friendly Physician - S2-E29
Plot hole: The scientist yells for Igor to capture the escaping Castaways, to which Igor responds "capture them yourself" in Ginger's voice. The scientist appears surprised to find out Igor is not Igor, but how can this be? He had to be the one to pull the lever to switch everyone's personalities and bodies around, so he should have already known.
Santa Comes to Visit and Stays and Stays - S6-E14
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. (00:09:30)