When Jonathan is talking with Charles as he's flipping through a photo album, he drains a glass of whiskey. The camera pans to Charles, and when it pans back to Jonathan, the glass is half full.
There is a closeup of Caroline's hand showing beautifully manicured and polished fingernails. No farmer's wife in the 1880s would ever have had nails like that.
When Mary is in bed in a near catatonic state, there is wayyy too much makeup on her face. So much red blush in the wrong place on her cheeks that she looks like someone slapped her. Not to mention the clumpy mascara and frosty underbrow eyeshadow. She would not have on ANY makeup if her breakdown was genuine.
Albert fell asleep in Quinn's barn full of stacked square hay bales. Square hay bales were invented until the 1930s in Pennsylvania. They would not have been in Minnesota in the 1880s.
When Charles, Caroline and Jonathan are at the table discussing Albert's whereabouts, Caroline mistakenly mentions Redwood City (which is in the Bay Area of California) as the city where the regional courthouse with Albert's records are, rather than Redwood Falls (which is only 38 miles from Walnut Grove). Karen Grassle, who portrays Caroline, may have subconsciously said Redwood City due to her being born and raised in the Bay Area of California, which is very close to Redwood City, CA.