Actor Allusion: In the two-part episode, "Brown Evil", Billy mistakes Hoss Delgado for a wussy video game character named Pat the Baker, both of which are voiced by Deiderich Bader.
Dean Toadblatt's voice actor is John Vernon, the same guy who played Dean Wormer in Animal House. This explains why he yelled "Gunderstank House!" after getting pranked on the episode where Billy and Mandy go to a Hogwarts-style school for kid wizards.
Fake Brit: Eris. What makes it funny, however, is that she's supposed to be a GREEK goddess. Furthermore, she also speaks like a Valley Girl.
Keep Circulating the Tapes: The first season was released on DVD but that has long since gone out of print, the same can be said for Big Boogey Adventure's DVD, several episodes were released either on Cartoon Network Holiday Compilations or as extras on other DVDs, thankfully the entire series (Sans Big Boogey Adventure, the Christmas special and Underfist) is available on iTunes. The show's first two seasons were also on Netflix, but are no longer there.
One of Us: Fictional example. In the "Brown Evil" episode, whereas Billy prefers playing a cheesy cooking game named "Pat the Baker", Mandy prefers a much more violent Resident Evil parody featuring Hoss Delgado. At the end of the episode, Hoss states that he never leaves home without the former example and is seen cheerfully playing it alongside Billy.
David Warner was the original voice of Nergal, but was later replaced with Martin Jarvis.
Dean Toadblatt was voiced by John Vernon in his first three appearances, but due to his death, in one later episode, Vernon was replaced with Ronnie Schell, who gave Toadblatt a much different hillbilly-like attitude.
Grim and Sperg have a few scenes together throughout the series; both of them are voiced by Greg Eagles.
Throw It In: Greg Eagles originally intended to voice Grim with a Swedish accent as a Shout-Out to The Seventh Seal, but it accidentally came out sounding more Jamaican and they decided to just go with that because it sounded funnier.
What Could Have Been: The Underfist movie was meant as a possible pilot for a new spin-off series (made a bit more obvious due to the new animation style), but Cartoon Network turned it down. Some sources say that the creator, Maxwell Atoms, pitched the idea as a prank. Atoms confirmed in aninterview that the rejection was due to changes in network heads.
In the episode "One Crazy Summoner", when Billy's parents are attacked by a giant anaconda, Billy's dad says the snake is trying to eat his Bismarck. Considering the fact that Billy and Mandy is famous for Getting Crap Past the Radar, this one sticks out.
You Look Familiar: Diedrich Bader as Hoss Delgado and Pat the Baker (a wussy video game character that Billy likes). Lampshaded in "Brown Evil", when Mandy and Grim say, "it's Hoss Delgado!" while Billy yells, "It's Pat the Baker!".
The show's Indian rights is with Cartoon Network India which does not air the original English version of the series.They first aired the series under the name-"Billy Mandy Aur Life Main Haddi" which was not a success. Then in 2014 they re-aired the show under the name-"Haddi Mera Buddy" which went on become the second most viewed show on Cartoon Network India. In this one they air Indian English version where it is dubbed by Indian dubbing artists.