I Knew It: In the first episode Max corrects Gwen that Ben had turned into an alien instead of a monster, leading fans to think he was involved with aliens sometime in his past, as it turns out he was part of a an intergalactic law-enforcement organization called the "Plumbers".
Word of God states that Humans (Homosapians) in the series actually have special powers all their own but it isn't as obvious as other species. They can interbreed with most other species, they are one of only a very select few who can us magic and manipulate mana, and they are the only species who have a form of Adaptability which makes them suited to live in various climates, terrains, and even outer dimensions with little to no effort.
At the very end of the where the elves try to get Grandpa Max to be Santa, you'll notice that the three children who talk to the old man look familiar. Who are they? Why, none other than Konohamaru, Moegi, and Udon from Naruto!
Originally, Ben would have transformed into HUMAN superheroes, with the show seeming to be more inspired by Dial H For Hero.
Ben's first design had him as a freckled redhead.
Gwen was originally going to be a classmate and one of his best friends, and she appeared in earlier concept art with a long ponytail and a pink shirt. She was changed to be a cousin since they may have been unable to figure out a decent reason for her to be on the road trip otherwise. The classmate aspect of this is referenced in the What If? episode "Goodbye and Good Riddance", where they become classmates at the end of the episode in order to better combat villains that might attack their hometown.
Cannonbolt was originally meant to be in the original set of 10 aliens, but was eventually dropped in favor of Diamondhead.
"Inferno" was considered as a name for Heatblast. Amusingly, this name was used for the Polish and French dub.