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.
Game Breaker: Hasten Time andO-Overdrive! The ultimate Hydrology spells, both achieved by Spell Synthesis (which can only be performed by Rosalian Mages and Imperial Scholars). Both allow you to instantly end the turn — for your enemies. Overdrive lets the caster then act five times, while Hasten Time lets the whole party get a move in.
Never Live It Down: In a sense; Galahad's mainly known for how you can kill him to get the Ice Sword.
It's even better in the The Ultimania Story, he is killed then revived by Grey, Galahad even says: "You could have just asked me for the sword"
Player Punch: The assassin in Jamil's scenario.The Remake even turns this into a full-fledged cutscene to drive the point home.
Another potential punch doesn't need a cutscene: "Alright, I'll just pop into Weston real quick and WHAT THE HELL IS THAT THING?!" Note that this one can happen even if the player isn't aware of the Jewel Beast beforehand, making it an even worse shock for unwary players.
Scrappy Level: The Isle of Evil. Features folding and moving stairs that accelerate the higher you climb; the slightest misstep sends you right to the bottom of the tower, forcing you to climb up again. Getting the timing right is... difficult. Oh, and if you leave, that's it: you can't come back. So you have to get everything done in a single trip.
Only in terms of getting the materials for it, especially the specific type of Mullock needed to complete the quest, you just need to spam Blunt Strike many times in battle (Blunt Strike does no damage and may paralyze the enemy)