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 majority of the women on the series had their set using hairspray as well using techniques that were not available in 1870s America. In fact hairspray was not invented until WWII and only came into regular use in the post-war years.
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.