In episode 45 "And Then I Wrote", Samantha goes into the kitchen to make sandwiches while Darrin reads her Civil War script. While she is in the kitchen, she has on a different dress and hairstyle than what she had on before she entered the kitchen.
This isn't a slip-up. Just before Samantha leaves she says, "Well, I'm going to change and then fix dinner". She probably did her hair at the same time.