The episode's official photos show characters in Halloween costumes, but there is nothing remotely relating to Halloween at all in accessible copies of the episode. In the original airing of the episode, the cold open features the warehouse turned into a haunted house, and Michael mimics committing suicide. Viewers were so extremely offended by the context, that the opening was then removed from all reruns and DVD copies, as well as digital streaming sources, like Netflix. The cold open can, however, be found on The Office's official YouTube channel. A search of "Koi Pond Cold Open" will return it as the top result.
Warren Lieberstein, a writer on the show and brother of Paul Lieberstein (Toby Flenderson), actually fell into a Koi pond, which is how he got the idea for Michael to fall in one.
This is the first episode that reveals Erin to be a foster child, although it isn't explicitly stated. During the conference room meeting discussing what is off limits for being made fun of, Erin's request to not be made fun of for being an orphan appears on the board. You can see it while Michael is writing Kevin's request to not be made fun of for his weight.
When the staff is making fun of Michael for falling into the koi pond, the background on Pam's (Jim's old) computer is an aquarium, further making fun of Michael.
To divert attention from his falling into a koi pond, Michael claimed to have been caught in a storm. Meredith states there couldn't have been a storm because her hip doesn't hurt. This is a reference to when Michael broke Meredith's hip in the fourth season episode Fun Run (2007).