Factual error: There's more PIR's (movement detectors used in security systems) in Hireclere than you could find in a security warehouse! Just look behind the actors in the corners of a room about 8 feet up. White plastic things about the size of a packet of smokes. I think the earliest units on the market started appearing around the early 70's. The PIR's in Downton Abby look to be around the late 80's onward - pretty advanced for 1914.
Episode #1.3 - S1-E3
Factual error: The Turkish character "Kemal Pamuk" is given the surname "Pamuk." However, surnames weren't used in Turkey till mid-thirties. Yes, Pamuk is a real Turkish surname but no Turk would be using surnames in 1912.
Episode #1.4 - S1-E4
Factual error: William plays a bit of music on the piano - the tune is called "After You've Gone", and it was written in 1918. This episode is set in 1913.
Episode #3.6 - S3-E6
Factual error: Thomas Barrow lights a cigarette with a disposable lighter which were not invented until the early 60's.
Episode #4.5 - S4-E5
Factual error: When Lady Edith visits a Harley Street doctor his nameplate is shown outside his surgery. His name is prefixed with "Dr." However, one of his post-nominal qualifications is "FRCS." As a Fellow of the Royal College of Surgeons, his name should be prefixed with "Mr.", as with all qualified surgeons in the British Isles.
Episode #4.7 - S4-E7
Factual error: When Edith goes to have an abortion, you can hear a Eurasian Collared Dove singing, but that species did not exist in Britain before the 1970s.
Factual error: Marigold can not possibly be as big as she is in this season. She's bigger than George, when she would only be a few months old.
Episode #6.3 - S6-E3
Factual error: During the wedding the groom and ushers are wearing boutonnieres pinned to their labels. Pinning boutonnieres to your lapel is a very new method. Traditionally the flower goes through the lapel button hole. This would still be a common use for the button hole at the time.
Answer: Word gets around. The staff are gossips amongst themselves, but those loyal to the family know who to keep secrets from and who they can trust to share in the knowledge.
Phixius ★