When Mr. Bast (Samuel West) first enters the Schlegel residence claiming his umbrella, he leaves the front door open. When he is invited to go upstairs immediately afterwards, the door is shut.
(at around 1h 15 mins) When the group drives past Howards End, it seems devoid of wisteria except on the right side of the house. Just a few moment later, as Margaret looks in windows, the wisteria seems to have grown over the front of the house.
When Charlie and Dolly Wilcox are hiding from Margaret Schlegel in the castle, the scene closes with low angle wide shot of the castle with a view of the sky behind it, revealing an aircraft contrail. There were no aircraft capable of leaving high-altitude contrails in the time period this movie is set in.
During the kiss with Paul, and during the music hall scenes, Helen is wearing a wristwatch. While wristwatches did exist at the time they were rare, and women normally wore a brooch type of timepiece. The wristwatch would not become common until the first world war, when they were given to soldiers to allow them to see the time while both hands were engaged.
In the last "farewell" scene, after Helen took her baby in her arms, she wanted to hold the farmer's boy's hand but instead, the boy, mistakenly, chose to wave to the Wilcox family members who were leaving the Howards End.