The phase of the moon in the show was meant to match up with the phase of the moon on the night each episode originally aired.

All of the names that Gregory (Collin Dean) names his lucky frog (Jack Jones) are: Kitty, Wirt, Wirt, Jr., George Washington, Mr. President, Benjamin Franklin, Dr. Cucumber, Greg, Jr., Skipper, Ronald, and Jason Funderberker.

In episode two, "Chapter 2: Hard Times at the Huskin' Bee", Gregory (Collin Dean), Wirt (Elijah Wood), and Beatrice (Melanie Lynskey) pass through Pottsfield, a town full of skeletons. This is a portmanteau of Potter's Field, a term for a graveyard for people who can not afford to buy a burial plot.

There were originally supposed to be eighteen episodes.

After the opening titles in each episode, a distant train horn can be heard. The boys meet this train in episode nine, "Chapter 9: Into the Unknown".

Based on the award-winning animated short Tome of the Unknown (2013).

The first mini-series created by Cartoon Network.

During the epilogue at the end of episode ten, "Chapter 10: The Unknown", Lorna (Shannyn Sossamon) can be seen reading a book called, "The Tome of the Unknown", which is also the name of the short story on which this mini-series was based.

Originally, the Unknown was supposed to be a Limbo world, and The Beast (Samuel Ramey) was supposed to be a guide for Wirt (Elijah Wood) and Gregory (Collin Dean) to bring them back to life.

In the soundtrack, it was revealed in "Langtree's Lament" that Jimmy Brown (Thomas Lennon) has only been gone for three days. However, Miss Langtree (Janet Klein) says that it feels like eight.

In episode five, "Chapter 5: Mad Love", one of the paintings in Endicott's mansion is also seen above the dining table in the Cartoon Network series Adventure Time (2010).

One of the episodes that didn't make the final cut was a man who makes dice out of the bones of kids.

Although Wirt (Elijah Wood) and Gregory (Collin Dean) arrive in The Unknown on Halloween and return to the real world only a few seconds later at the end of the mini-series, it was originally planned for the boys to come back on Christmas day.

The Tavern Keeper (Audrey Wasilewski) has been believed to be based on Betty Boop.

At the end of the mini-series, Lorna (Shannyn Sossamon) is shown reading of book titled, "The Tome of Unknown", which is the name of the pilot.

Creator Patrick McHale came up with the original concept for this mini-series in 2004.

The character of Gregory, the younger of the two brothers, was based on Executive Producer Patrick McHale's son.

Although being the main antagonist, The Beast only appeared in four episodes.

All of the main characters of this mini-series are briefly shown in the introduction to episode one, "Chapter 1: The Old Grist Mill". One can even see a launching of the boat on which the brothers will travel in episode six, "Chapter 6: Lullaby in Frogland".

Episode six, "Chapter 6: Lullaby in Frogland" was originally supposed to be a two-part episode.

A painting in Endicott's mansion is seen in a hallway portraying the Beast.

The soundtrack for the show is one of the most requested. The mixtape, "For Sara", from episode nine, "Chapter 9: Into the Unknown", was available on cassette for a limited time (now available on Soundcloud), and the full soundtrack, on vinyl only, is soon to be released for a limited time (Executive Producer Patrick McHale actually unboxed it on his YouTube channel). The full soundtrack is going to be released on Soundcloud.

The Beast's true appearance is visible briefly and only during the climax.

As the camera pans out from the hospital at the end of this mini-series, you can see the city in which the brothers live. It has a very strong resemblence to a city from another Cartoon Network show, Clarence (2013).

Wirt was originally named Walter.

The bell Auntie Whispers (Tim Curry) used to control Lorna (Shannyn Sossamon) resembled Lorna.

In the episode "Lullaby in Frogland", when Wirt is looking at the angry crowd, there are two frogs resembling Luke and Professor Layton in the crowd

Originally planned to be a regular television series.

Greg and Wirt are half-brothers.

In the dream scene in episode eight, "Chapter 8: Babes in the Woods", a pink bunny can be seen hitting a drum, resembling the Energizer Bunny.

Christopher Lloyd and Elijah Wood appeared in Back to the Future Part II (1989).

Auntie Whispers was played by Tim Curry, despite Whispers being female.

In episode nine, "Chapter 9: Into the Unknown", Gregory (Collin Dean) hides behind Quincy Endicott's (John Cleese's) gravestone.

In the opening, Lorna (Shannyn Sossamon) can be sorting a large number of bones that just so happen to be her victims.