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.
Complacent Gaming Syndrome: Averted due to the experience system detailed above. If you keep using a mastered item, you're missing out on EXP and slowing your leveling to a crawl. You have to constantly switch equipment to continue levelling up.
Demonic Spiders: Monstrosities in general. High health and nasty attack patterns which deal very high damage and cannot be parried.
Especially in the negative rebirths, where there could be some nasty compulsory XP bonuses mashed into a monstrosity like 'Do 20 scratches', 'Kill Titan with scratch',... Good luck.
Game Breaker: Heal magic can be this, particularly with the last magic ring as it gives you full health and can recharge several times in the course of one fight. Then again, considering the requirements to get it, it can also be considered as Bragging Rights Reward.
Discover Bloodline -1, and you will gain the strongest set of equipment, able to far outstrip even the Infinity Blade itself. You will smile with glee when you get to strike down the God King in mere seconds!
In II with the Vault of Tears expansion, the Holy Band is definitely this. It does 10000 irresistible holy damage that can also heal you the same amount as the damage being done.
Alchemy in III brings us the Rare Health Potion: cheap to make (two blue berries and two butterflies), only 7 minutes to cook, and restores 250 health per second. There are very few enemies capable of matching this output in damage on you, making most fights trivial.
Good Bad Bugs: You can retry the fights against the bosses by exiting the game and killing the app before they run you through. Has never been fixed, likely due to the outcry it would cause.
That One Attack: The God King's Fury attack can very well end a Bloodline then and there. It's tricky to parry, very difficult to dodge, and is capable of tearing through your entire health bar. It gets even worse in the final round, when he starts using it every other attack pattern.
That One Boss: The God King is an intentional example of this trope. His lethal powers cause the entire storyline. He starts at level fifty when the player is level one. Every time the player beats him, he goes up another fifty levels.
The sequel has a few of these, naturally, though the one that takes the cake is the Bog Giant, the tree trunk horror with the Vile Blade stuck in it that comes with the "Vault of Tears" expansion — thankfully, you can restart the fight if you die like a regular encounter. But it is horrifyingly fast for something so huge (you will need to get used to its speed to dodge and parry properly), has new attacks that can catch you off-guard, and when felled, it goes One-Winged Angel and transforms into the Moss Golem, a Monstrosity that will give you a hard time even if you mastered fighting the first form. Buffs used in the first battle will disappear at the subsequent battle and there is no break between them. However, you will be rewarded with a sword that rivals the Infinity Blade in attack power after that.
The third game has Therin. He uses the same Blade on a Stick style as the Fel Siren, an enemy considered to have one of the harder attack patterns to fight against... only Therin likes to do a lot of feints, tricking you into doing the wrong counter all the time.
Also in the third game is Ryth. He's always overleveled to the point that one slash will kill you, and your gems and potions are rendered useless against him. You also have to use a Light weapon the first time you fight him - one of his attacks can only be blocked, so he'll take out your shield very quickly.