In the episode "Billy Gets an A", Billy using the scythe's magic to change the grade on his exam somehow creating a Bizarro Universe, which affected all but Grim and Mandy. Initially, I thought the lack of change was to give Grim someone else to snark with. However, it wasn't until later that I realized that Mandy had already manipulated her bizarro-self into being just like her in an earlier episode. -Pinkbaron
In one of the show's first episodes, the kids' life hourglasses get flipped over and they rapidly regress in age. Irwin becomes a baby and pops out of existence, but Billy and Mandy both regress to fetuses before disappearing. It wasn't until years later in the series finale that we learn Irwin isn't human.
At first, the character's continual deaths seem a sign of Negative Continuity. Until you realise that they have the Grim Reaper under total control. The being responsible for death itself! Grim has to keep them from staying dead in order to fufill his end of the bargain, or bring people back to life when Billy and Mandy wish it. Even when he is killed off, he's fine since it's impossible to kill death.
They could wish for eternal life like Jack did in the Halloween episode.
Except all the other characters that get killed/zombified/whatever. This show is made of Negative Continuity.
Being Billy and Mandy's whipping boy would make it very hard for Grimm to do his job. Their crazy adventures means Grimm didn't have the time to off them.
It's hinted in a few episodes that Grimm isn't allowed to reap without permission from Billy and/or Mandy.
While a middle school grudge is one thing, Grim and Boogie's animosity made more sense once we got more backstory. Boogie hates Grim for winning the election to become the Grim Reaper, while Grim not only scared him, which is what caused him to win the election, because he was Boogie's most frequent victim, but also because Boogie cost him his only friend, Velma.
Yogi and Boo-Boo being afraid of caves just seems like a joke. However, brown bears really are afraid of caves.
At the end of the episode with the wishing skull, the skull, Thronambular, grants Grim's wish for both to be free of the respective bargains. Thronambular screws Grim over, leaving him trapped as a skull forced to grant 8 more wishes, while Thronambular is free in Grim's body. While the episode ends there with Negative Continuity, seeing as Thronambular now has to put up with Billy and Mandy, it's not unreasonable to assume he couldn't take it, and wished him and Grim back to their respective positions.
When Billy grows enough hair to be mistaken for a Sasquatch, Gladys mistakes him for the actual Sasquatch and chases him out of the house, implying that she had to deal with the Sasquatch as a Stalker with a Crush some years ago. However, we never actually see proof that the Sasquatch exists in this universe. We do, however, have proof, as of "Here Thar be Dwarves," that Billy's dad likes to dress up as the Sasquatch and fool his family…
The ending of "The Greatest Love Story Ever Told", with Junior and Irwin slow dancing together, has been mentioned as one of the show's prime examples of Ho Yay. However, it takes on new meaning when one remembers how the two first met. Junior, seeking friends and popularity, saw Irwin fawning over Mindy in a distinctly romantic manner. Who is the next person Junior takes on the form of?
Fridge Logic - In canon. Irwin's dad explains how he met and fell in love with Irwin's mom, who happens to be a mummy.
Irwin's Dad: Nobody can tell you who to fall in love with, but we've managed to make it work all these years. Leaving a whole lot of questions that don't need to be answered.
Mandy: Eh, works for me.
Grim: Me too.
Billy: ...But how did you and Irwin's mom...
Irwin's Dad, in the exact same tone: Leaving a whole lot of questions that don't need to be answered.
Oh but it does have a answer... Irwin's father is Dracula which makes him a Dhampyr and since it makes him a half-monster and mummies are monsters then that explains the compatibility, of course we don't know this until "Dracula Must Die!"
In the episode "Chocolate Sailor", Billy turns completely into solid chocolate after eating too many supernatural chocolates. If that isn't enough, Billy even begins to eat himself, saying to Mandy that "...it's not that painful" at one point, and continues to do so, reducing himself to "his big, chocolate head" near the end of the episode. At the episode's climax, the Chocolate Sailor gives Billy the antidote, but with a twist: it's in a box of assorted chocolates and Billy has to pick the right one. Since Billy is pretty much stupid incarnate (the antidote bar is called Antidote), he eats the whole assortment and promptly explodes. The episode ends with Mandy and Grim eating hot fudge sundaes, with them saying Billy is in a better place. The bottle of hot fudge syrup they have then says "I like chocolate!", basically implying that they scooped up Billy's remains and dumped them into a chocolate syrup (and to add that Grim and Mandy are eating hot fudge sundaes...)
Whether they know it or not, they couldn't care less anyway.