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.
YMMV: Brave Soul
Critical Research Failure — Nobody drinks a cocktail that is almost 100% alcohol and gets to live. Even if it DOES taste like juice, you'd be too busy screaming in agony to notice.
Game Breaker — The sword called "The Fastest" is arguably this. It has low attack power, but its extreme speed boost makes you zoom across the landscape (to the point where you become somewhat difficult to control), Spam projectiles, Spam Attack melee attacks, run circles around fired projectiles and slay most enemies before they even know you're there.
The Fastest + anything qualifies, really, as it always ratchets up your attack and move speeds by a huge amount (usually to the max). The only thing that it doesn't go well with is the Speed Limit Talisman, but why would you do that in the first place?
The Developer's Room, available from the second playthrough onward, sells every single item and equipment in the game save the "ultimate" weapons and armors, as well as the Lightsaber. You can buy the Sword of Desire — which converts earned experience directly into gold — for 2000G, which you usually have by the time you finish the tutorial missions (in addition to the cash purse from Jim). Why is this useful? The same shop sells The Fastest (which above duh) and the Oricon Sword (dropped by Gale, the only other high-end sword without stat drawbacks or asinine requirements and a great alternative to the Fastest), for 50,000 and 60,000 Gold respectively. The Fastest is universally acknowledged as a Game Breaker among Game Breakers, and you can buy it from the in-game shop.