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: Metal Raptors and Death Spiders in Omnitopia. The raptors are agile and difficult to hit, and the aptly-named Death Spiders can kill you with one bite.
Game Breaker: The Aura spell grants total invulnerability for a short time. Also, the bazooka, which is immensely more powerful than any other weapon in the game, doesn't require leveling up, and can easily be provided with infinite ammo. Even before those, you get the Crush spell, which becomes ridiculously over powered in a few levels, and can defeat bosses, even That One Boss, in seconds.
Just outside the entrance to Nobilia is a place where the boy will spin around. If you stand here long enough (after he complains about spinning), he'll sink down and run into a place where there's 99 spice and rice. And this resets when you into town and come back out. Since spice and rice are the two basic commodities used for trading, coming into town before the first time (if you do, the timer starts before the market closes) and basically do everything in the market in the 20 or so minutes you have. You can also just use this to trade for more valuable items and sell them to the appraiser. The only downside is that the boy takes damage at a rate of 1 per second, so stock up on healing.
Energize + Fully-leveled Neutron Blade = 3-4 hits of 500+ damage within 2 seconds. The Spell almost instantly recharges you to charge level 3, allowing you to do it again. And Again. And Again. And Again. Add Horace's Spell mentioned above to make you invincible while you spin-slice the enemy to death.
However, what keeps it from completely breaking the game is that it's obtained fairly late. It basically only breaks the Final Boss battle.
One-upping Aura is Barrier, which does the same exact thing, only it heals both characters. While it's probably superseded by Super Heal, if you get Barrier high enough, you may not need it anymore. The only problem is that it's easily missable - there's a very small window of opportunity to get it (you need to speak to Horace before fighting Tiny).
Toaster-Dog comes very close if you leave it controlled by the AI. In addition to ludicrously high critical hit rates and a new ranged attack, Toaster-Dog will also randomly counterattack with a full-power attack when he blocks/dodges. You barely need to do anything in the surface tubes sections: just let Toaster-Dog take care of it.
Good Bad Bugs: You can skip the two Rogues because the exit behind them is not actually blocked. There's a way to get around the Verminator. However, you must remember not to try and use the door he normally blocks, or else you'll be stuck in his crates.
The game is actually chock full of these. The Sprint Meter example below is technically a bug, as is the aforementioned Bazooka infinite ammo. Also, the Silver Sheath accessory is technically worthless since every sword-type weapon already has its bonus applied. Evermore is a veritable haven of relatively benign bugs and glitches.
A little glitch involving the attack and defense buff spells and resetting the game can get your attack and defense stats well over sixty-five THOUSAND, as shown in this part of a Let's Play made by My Name Is Kaz and Medibot.
This glitch takes advantage of overflow (in the opposite direction). Since the buffing spells raise your stats, rather than become a modifier in the damage calculation, it's possible the game simply subtracts those added stats and the effect would be so great that it would overflow back to 65535. But the game does remember your base stat for some reason.
Infinite Call Beads! One square in Nobilia, if done properly, will yield infinite Call Beads (you can only carry 99 though) if you continue to push the button while standing there.
Nightmare Fuel: The monster Mungola first appears as a pair of sickly yellow eyes behind a curtain. They're the size of car tires.
That One Boss: The Verminator, a Rodent of Unusual Size that perches on a stack of crates (safely out of reach of most physical attacks), hurls potent alchemy spells including Explosion and Plague, and occasionally robs you of HP with Drain. You'd better either have leveled up your spear and offensive alchemy or be willing to use up some Call Beads.
Much earlier in the swap, doubling as a Wake-Up Call Boss, Salabog hovers over the swamp out of range of your normal weapons, and as above can only be damaged through usage of a leveled-up spear or alchemy. There are two main differences—first, because he's the third boss of the game, Salabog is fought at a time when you have very few alchemy formulas at low levels, whereas Verminator is fought late in the game when you've powered your alchemy up. To make up for being out of reach, occasionally Salabog will snap its head forward to come in range. However, this also hits you for some nasty damage, so get out of the way.
Salabog will also spit fireballs out at you that home in, inevitably screwing up your attempts to hit him at least once or twice.
The clones you fight after the first section of the forest maze each have an alchemy spell. The first one uses Flash, the second uses Crush, but the last one... The last one has an end game spell called Nitro. Nitro will basically take a good two-thirds of the boy's life.