When Steven barges into Howard's office, he is shown starting to close the door, followed by the sound of a door closing. However, in the subsequent shot, the door is open again.
When Mary and Steven are chatting on the park bench, the cigarette switches from being in Mary's mouth to being out of the frame between shots.
At 24:00, Howard is using a vacuum coffee maker with a cork seal - although the burner is still on, so the coffee would not have returned to the bottom beaker while the heat was still on.