The color of Matthew's hair changes periodically. At times it's almost brown and others a lighter blonde.
There is inconsistency in the men standing when a women enters or leaves the room. Sometimes when just the family is at dinner, a woman will leave the table and the men do not stand. There is also inconsistency in the staff standing when Carson enters and leaves the servants hall. Sometimes they do, sometimes they don't.
Robert and Cora Crawley both have blue eyes, but their daughters Mary and Edith have brown eyes. While it's not very common, two blue-eyed parents can produce brown-eyed children. Eye color is a complex trait that depends on the state of several interacting genes. The OCA2 gene on chromosome 15, which usually determines eye color, comes in different strengths. A person with a weak form of the gene will have blue eyes, and a person with a strong form will have brown eyes. Individuals also have other eye-color genes. For example, if one of these lesser genes is strong, it can make the weak form (blue) of OCA2 work much more effectively. Depending on the interactions of other genes, the resulting eye color can be any shade of brown, hazel, green or blue.
There are several occasions in the first two seasons when a wedding band tan line is evident on Carson's ring finger. Jim Carter who plays him is married, but Carson was not.
In episodes showing a Christmas tree inside Downton Abbey, classy all-white Christmas lights with tiny raisin-sized bulbs are lit; however, these impressive lights didn't appear for sale until the late 1960s and early 1970s. In the series' historical period, the smallest bulbs available were Mazda Lights, which are golf ball-sized.
For the most part, none of the aristocratic characters in the series speak with the proper received pronunciation that would be accurate of the aristocracy of the time.
Downton Abbey is intended to be situated in the York Plain of North Yorkshire - characters occasionally refer to Thirsk and Ripon as being local towns. However in scenes of the village streets of Downton, the buildings are made of sandy Cotswold stone (as found in Bampton, Oxfordshire where these scenes were filmed), unlike the darker types of stone that are generally found in Yorkshire.
Lady Sybil, Lady Edith and Lady Mary are full biological sisters and yet they look almost nothing alike.