Spoiler warning!
This article contains plot spoilers. If you do not yet know of this information, it is recommended that you read at your own risk or not at all.
At the beginning of the mission, if you talk to Lydia, she will tell you that someone knocked on the door of the Hounds Pit Pub a few days ago which gave everyone a scare. Talking to Cecelia reveals that she did it as a joke.
When Corvo enters the Void for the first time, he can see a school of hagfishes swimming around. This marks the only time in the franchise, animals - other than whales or human - appear in the Void.
The rats you summoned don't attack Granny Rag and instead just stand around.
If you possess Granny Rag, she will say "Where? What's this. So confused. Oh, has he come for me at last? At last! At LAST!" (Yes, it's possible to gather enough runes in this mission to possess humans, and yes she says the same thing if you possess her in the next mission)
The sick wolfhound, Voracious, has much higher health than normal wolfhounds. It usually takes a single bolt or bullet at point blank range to kill a hound but 4 bolts or 3 bullets are needed to put down Voracious. Also, Voracious's barks don't alert the Overseers like other hounds, probably due to the fact that they know he's sick.
There is a stark contrast in term of atmosphere and interior design between the first and second floors of the High Overseer Office. The first floor is cold, austere, imposing, and mostly constructed with stones and granite. It's designed to intimidate people who visit the place. However, on the second floor where it's off-limits to the public, we start to see wooden panels, wooden floors, and red carpets. The meeting room of the Higher Overseer even has drapes, cushioned chairs, and a fireplace. [Credit: sckchui]
There are a few interesting details regarding the epitaphs and urns in the Office of the High Overseer:
High Overseer Tynan Wallace was the only one who did not die a painful or terrible death.
Benjamin Holger is said to be killed by a knife driven through his eye during the Siege of White Cliff. One of the eyes of the Benjamin Holger bust in front of Campbell's secret chamber is replaced by a gem, referring to that fact.
High Overseer Francis Perry went missing during the Fugue Feast and was never found. If you break his urn, there is actually no ash inside.
High Overseer Scott Grafton was cremated with Holger's Opal and you can break the urn (or look down into the urn's mouth) to get the opal, netting you 200 coins.
On the second floor of the High Overseer office, there are two stone murals depicting two men, though I'm not sure who they are.
Many people wondered why the Overseers just exiled Thaddeus Campbell without any hesitation once he was branded. This is because Campbell's journal, the so-called "Black Book", contained incriminating information on many of the Overseers, and Teague Martin said that Campbell used that to black-mailed high-ranking Abbey members into supporting him. It is later implied that Martin himself used the same journal to gain the position of High Overseer.
