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.
Anticlimax Boss: Certain mice are easy to catch and yield enormous rewards of points and gold, but getting into position to catch them is very difficult and requires a lot of time and expensive cheese.
The Eclipse Mouse, if you have the right weapon. With a description stating that it is darkness and fear itself, it sure doesn't put up much of a fight if you wield the Ancient Box Trap. In fact, his difficulty rating is "Effortless" is you do so — the lowest possible.
Harpy Mice in the Lagoon. Players would've most likely brought either a Physical, Tactical or Hydro trap, while their main weakness is Shadow, and their mouse power is very high. The Harpy's description even Lampshades this:
"Hunter's lore describes these mice as a curse, bringing bad luck and fruitless hunts. Hunters lament at the eerie caw of a Harpy mouse descending from the clouds, knowing that escape is impossible. Harpy mice exist to torture Mouse Hunters by constantly stealing their cheese and raiding their supplies. Their swift wings bring a dark plague of frustration to any hunter unfortunate enough to encounter them"
Vampire mice in the Mousoleum and the Catacombs. They give you very little gold and points compared to the other mice there. If they happen to steal your gold, they steal a lot more than the other mice.
Fridge Horror: The Warmonger mouse sends in the new recruits on the first wave before sending in the more experienced soldiers on the next two. In short, the inexperienced mice who wanted to join (or were forced to join) his army are literally being sent to an early death!
Many Limited Edition traps have respectable Power and stupidly high Luck. Luck also determines whether or not you succeed in catching a mouse, and the effectiveness of luck rises exponentially. Of note are the Partybot (Physical), Chrome Deathbot/Drillbot (Physical), Giant Speaker/2010 Blastoff/Rewer's Riposte (Tactical), and Kraken Chaos/Double Diamond Adventure (Hydro)
The Oasis Water Node trap, the Sphynx Wrath and especially the Sandstorm Monstrobot trump all of these. However, if you want those three traps, you will have to Earn Your Fun.
The Phantasmic Oasis trap, Grand Arcanum trap and the Clockwork Portal trap also count.
Internet Backdraft: In the past, when the name Ronza was mentioned, the forums would go crazy. Nowadays, there's no such insanity, but it continues to be parodied.
Scrappy Mechanic: Cursed City Minigame. It requires the player to actively switch between 3 charms after each successful catch to dispel 3 curses to allow another mouse with better loot to be attracted, but Corrupt mice can waltz in at anytime and reset all the curses, even if he's caught. After successfully dispelling all 3 curses, yo uare given a grace period of 3 hunts where Corrupt can not be attracted, but after that, Corrupt will reset all your work when attracted. Some take days to even manage dispelling all 3 curses.
Made worse when you're hunting for the Dark Magi boss. He can only be attracted after dispelling all 3 curses and equipping a charm, and that charm will also cause only one other mouse to be attracted: The Corrupt.
Tear Jerker: Greedy Al is an embittered Jerkass by the time you confront him during the Great Winter Hunt, but the letters to Santa he wrote when he was young and disappointed by empty stockings are heartbreaking.
Depending on whether the "Devs Hate U", each run in the Zugzwang's Tower can be a big failure, or success, or anywhere in between.
The Dreaded Horde in the Jungle of Dread are made to be especially hard. Even with the best traps for that location (such as the Cackle Lantern and the Clockapult of Time), you will fail to catch them over half the time.
Nightmare Fuel: The painting of their father is moody throughout the film, but then magically changes to a grin, when the sons finally work together for real. In intention, a beautiful event to show the father being proud of his sons even after death, but the goddamn painting changes through supernatural forces. Not intentionally scary, but if you think about it, it's chilling.
One-Scene Wonder: Christopher Walken has an extended cameo as an expert exterminator, who, of course, fails to take down the mouse. This being Walken, it goes without saying that the character is an eccentric Large Ham who takes to pest control like a member of a SWAT team.