When Daphne is promenading with Hastings, she asks him to button her cuff. When the shot changes to show Daphne, her cuff is fully buttoned.
In one scene, Benedict and Eloise are sitting on swings, smoking cigarettes. The cigarettes look to be modern day cigarettes which could not be the case in the 1700s or even early 1800s. Cigarettes of that day were hand rolled. The cigarettes seen today came about in the 1880s in America.
In an early scene, the queen is watching a contortionist, who is wearing stilletos, which were not invented until the 20th century.
Lord Hastings is shown boxing in a boxing ring. These were not used until the 1830's.
The scene with Lady Violet Bridgerton and her son Anthony in front of the fireplace, it's clearly a gas fireplace. You can see the row of flames under the logs, uniformly and evenly spaced. Gas fireplaces were not produced until 1895.