Billy's butt was showen several times, and it didn't have a birthmark.
After El Uglio removes his mask, Mandy's costume disappears.
El Uglio calling Billy "hermano" is a reference to the Spanish language. In Spanish, "hermano" means brother.
While telling the story of El Dia de Los Muertos, the boy's hair turns black.
Harold: Sure, Mandy, we'll call it Mexico.
Mandy: No. This IS Mexico!
Billy: (to El Uglio) Can you sign my face and/or butt?
El Uglio: Hermano!
Billy: No, Billy.
Mandy shows rare emotion, as she did in Billy & Mandy's Big Boogey Adventure. But as where then her eyes shimmered, here we actually see tears in her eyes.
Judge Roy Spleen and Fred Fredburger made cameos.
The story of the origin of Dia de Los Muertos that the boy tells is based on the conquisition of Hernando Cortes in 1518. The true origin of the holiday takes place in such cultures as the Mayas. Rituals celebrating the deaths of ancestors have been observed by these civilizations perhaps for as long as 2,500 – 3,000 years.
The opening credits of El Dia de Los Muertos Estupidos is in Spanish.
For some unknown reasons, El Uglio is Billy's "twin" brother. They don't look anything alike and they're not the same age.
They could be fraternal twins and his twin could look old for his age.
Before Heartburn started, a "deleted scene" of Keeper of the Reaper is shown. The scene was when Billy called Mandy's parents to the stand and they say they don't appear in a lot of episodes because of their fear of Mandy. After that, they start making out.
This is the second time in the series Irwin and Mandy kiss. The first time was in the TV movie, Billy & Mandy's Big Boogey Adventure.
It is discovered that when Irwin was a baby, he behaved very badly, most likely because he is half vampire and half mummy.
Possibly the series finale.
Irwin: My heart grew 3 times in size that day.
It is a reference to How the Grinch Stole Christmas, when the Grinch's heart grew 3 times in size. There was also a reference to How the Grinch Stole Christmas in A Grim Surprise when Grim's heart grows 3 times its size.
The churro dogs are real, except they're a type of dessert the churro minus the dog. A churro is a fried pastry, often eaten in Spanish countries as kids' breakfast. They are often eaten with hot cocoa and topped with sugar.