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.
Nightmare Fuel: The part where the Mechanical Spider comes alive. Just seeing the Spider's jaw open and glimpsing a raging furnace with broken machine parts inside will DEFINITELY call for a change of pants. No wonder Max wanted to implement that crossover switch so badly.
Ear Worm: The exaggerated Muzak that plays on the Bureau's Security Face will definitely get inside your skull if you don't know what you're doing.
Technology Marches On: When you're watching the sendoff party video, look closely at the phone Lilah & Max's boss is holding.
In the chemistry lab puzzle, there's a TV-like device with two items next to it that look like floppy disks.
That One Puzzle: A few puzzles are quite hard, like the Fire Puzzle in the spider realm or the programming puzzle in the Bismuth realm, but the very last one takes the cake: 16 yellow buttons guarding the crossover switch, half of which are fake and if you click any of those, they move and change their positions. You're supposed to hit the buttons that did NOT move, but because all of them look the same, it's VERY hard to keep your eyes on the right ones! Worse, there's no set pattern, and each movement of the camouflage buttons is random. The Conductor almost mocks you about this after you solve it and flip the switch to human control.
"...The camouflage I placed upon the toggles has delayed you long enough."
Leaning on the Fourth Wall: Inside the statue of Bismuth, each painting on the 3 dream realms has four sketches corresponding to them. These sketches are most likely real ones made by the development team.
One sketch of the Bureau realm shows the Chief in a much different design, and smoking a cigarette that, according to the hint book, would have been used in the same fashion as his glasses that pass through the screen in-game.