Accidentally Accurate: How accidental is up to debate, but the show's version of Dracula is actually pretty accurate compared to the original novel, especially in terms of appearance. They even got the Daywalking Vampire thing right!
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. Mainly, though, her mannerisms were meant to be a reference to Madonna.
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 "Nigel Planter and the Order of the Peanuts", Vernon was replaced with Ronnie Schell, who gave Toadblatt a much different hillbilly-like attitude. The change was lampshaded by one of the students at Toadblatt's, and the change may have been used to make reference to the fact that the same thing had happened with Albus Dumbledore in the Harry Potter films.
Grim and Sperg have a few scenes together throughout the series; both of them are voiced by Greg Eagles.
Irwin's family is full of this. Irwin's dad, grandmother, and grandfather (Dracula) are all voiced by Phil Lamarr. Vanessa Marshall voices Irwin's mother and Irwin himself.
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 his interview with Rebel Taxi that the rejection was due to changes in network heads.
Grim was initially to have a British accent and a personality similar to that of Dr. Smith, even right down to being voiced by Jonathan Harris himself!
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.
Both Billy and Mandy's personalities were different at the time too. Billy was less stupid and instead more shy and gullible, while Mandy was a creepy Perpetual SmilerGenki Girl. In Atoms' college short film, Billy is actually smarter than the final product while Mandy initially seems nicer...but then she tricks Billy into drilling a hole into his skull, which prompts him to give a big stupid grin.
For the Codename: Kids Next Door crossover, Mandy was originally going to torture Numbah One by forcing him to watch Sheep in the Big City. But CN's legal dept. feared that Sheep's creator, Mo Willems, wouldn't find it funny (as he was still working on KND at the time), so they had to clear it with Willems himself. He had no issues, but the legal guys still said no. So they tried changing it to Evil Con Carne, but the legal team said no to that too, for whatever reason. So instead we had Fred Fredburger torturing Nigel.
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.