Although it could have been adapted to activate an electric doorbell in modern times, the button next to Mr. Collins's parsonage front door - seen when Mr. Darcy exits following his unsuccessful proposal to Elizabeth and identified as belonging to an electric doorbell - is clearly the pull-knob of a mechanical bell, attached to a wire inside that travels through pulleys to activate a mechanical bell on the wall.
In the drawing room at Longbourn just before the soldiers are
to leave Meryton, Mrs. Bennett is talking to Col. Forster with Mr. Bennett and Wickham looking on. At one point we hear the conversation happening, but the shot is from another time when they were not talking to each other.
When Wickham and Lizzy are walking the grounds at Longbourn in episode 3, Wikcham tells Lizzy that Darcy's sister is sixteen years old. But when Lizzy and her aunt and uncle visit Pemberly (in the summer the next year), the housekeeper shows them a painting of Georgiana painted for her sixteenth birthday earlier that year.
When Darcy comes back to Rosings after his unsuccessful proposal, we see him go up the stairs to his room. In the next scene, after he starts to write the letter, he goes to the window, and when the camera zoom out it seems that the room is on the ground floor.
After Darcy dives into the water at Pemberley and encounters Lizzy there, his hair is wet when they speak. Only a few moments later, he hurries out of the house to see her again, and his hair looks dry.
When Mr. Darcy first appears at Pemberley, the shadows of the actors are quite long, as if towards the end of a day as the sun was low in the sky. Yet later, after Mr. Darcy has changed clothes, the shadows are far shorter, indicating the sun was higher and it was earlier in the day.