The very possible possibility that there has been a long string of missing children's cases all due to the fact that at some point, some poor innocent kids entered the haunted mansion and weren't competent enough to escape the hungry child-eating grasp of Fangula, Mums, and Frankenbeans.
And if the Protagonists hit the wrong combination of notes on the pipe organ they'd be three more "missing" children.
We never find out exactly what Les Fortunes was imprisoned for, but it was something serious enough for him to be put in solitary confinement. It does seem pretty likely that he had murdered someone.
It's easy to forget in how dangerous Nukus releasing Les makes him, but the Crustaceans release an entire prison's inmate population, presumably including the ones in solitary confinement for life like Les is, and a good number of them are shown escaping.
In one episode, the Hillhurst monsters are plagued by Wolfie's fleas. All right, he's a wolf/human hybrid, the Beetleborgs are human children, Frankie and Fangula both have some sort of undead human blood, but why do they bite Flabber? ...well, he's some sort of seventh-dimension entity...WHY DO THEY BITE MUMS? A mummy literally only exists because it's not exposed to liquid. Mummies don't produce body heat, so why would fleas even TRY to bite them?
Why do the props and effects constantly change throughout the series with no explanation- ie. the Sonic Lasers' fluctuation between different color schemes, and the changing sizes of the AVs / BVs? If they were taken from a comic book, there's no guarantee that Art Fortunes would draw and/or color them correctly in every issue of the comic, so hence their appearance could vary from issue to issue or even frame to frame. So Flabber's magic, in trying to bring the comic stuff to reality, duplicated those inconsistencies.
Nukus not destroying the Beetleborgs after he destroys their powers seems foolish, until you realize his plan. With the Beetleborgs neutralized he then went after Flabber to insure they remained harmless. This strategy is used a few times in later episodes.
Vexor being a Non-Action Big Bad makes sense when you realize the Magnavores came from an unfinished comic series and is the Big Bad. He very likely didn't have many if any fighting scenes before he emerged into the real world.