These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Demonic Spiders: Slithers are sometimes this, sometimes just Goddamn Bats. A more traditional example would be the spider Darkling that spews a bunch of tiny spiders when it gets hit with Resplendently Scintillating Illustrations.
Game Breaker: Essentially the Flame Bringer power. Technology, electricity, and fire aren't supposed to work during Midnight, even if a Midnighter touches it. Unless they're a Flame Bringer, like Jessica, who can break that rule on a grand scale. She can also give 13-letter names to much larger objects than the others. Which means even though Jessica was completely useless for most of the first book, in the second and third she can kick some serious Slither and Darkling ass. "The all-powerful Flashlight Bringer" indeed.