The fashions worn in the series reflected the 1960s filming era rather than its 1880s setting. For example, a proper woman such as Victoria Barkley would have never worn pants and nearly all of the men's clothing is period inappropriate.
The goof item below may give away important plot points.
In the episode The Big Valley: Caesar's Wife, Audra is pouring a half a pitcher of lemonade into Will's (Tim O'Kelly) cup. Being startled Audra goes to investigate with Will outside the outside barn; Audra is still carrying the pitcher of lemonade, but it's still full.