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.
Accidental Aesop: The original Japanese script involved a smuggling ring of illegal opium in Edo. The 1991 official English localization could not mention such drugs, so changed opium to "bananas". An NPC lampshades this by asking why bananas have to be illegal in the first place. It's obvious to most players that criminalizing bananas is silly, and the sheer organized crime involved might not exist without a legal ban on bananas. In the real world, this is an increasingly vocal argument against the War on Drugs, especially after a 2011 United Nations commission declared the international War on Drugs to be a costly, violent failure — drug crime and drug violence are usually caused by drug bans, not vice versa.
Crowning Moment of Heartwarming: Quite a few for such an early RPG. But one that stands out is the hero and his/her/its whole family going on an adventure together at the end of the game.
Game Breaker: The Excalibur. It deals a minimum of 1050 damage (before defense and weapon resistances, of course), hits a whole group, never misses, and unlike other weapons, never breaks.
Good Bad Bugs: A certain enemy, when inflicted with the confuse status, will attack itself with Punch for upwards of 6000 damage (Punch does more damage the fewer uses of it are left, and the enemy is using a Punch it doesn't actually have).
An infamous bug involves you teleporting off the Dragon Racetrack while on the dragon. This completely breaks the game as you can walk through walls, allowing you to visit ultra high end dungeons early, steal helper NPCs from the end of the game, etc etc.
There is a way to give your robots infinite agility. Equipping a martial-arts ability on a robot boosts their agility. Completely use up a martial arts ability that's equipped on a robot, and even though the item is gone, the game won't recalculate the robot's agility and they'll keep the boost.
Nightmare Fuel: Apollo's demise. How about the "silently screaming" faces in the walls of the not-quite Final Dungeon?
The arrangement of the dungeon theme used in the remake. It begins with shrill, descending notes and breaks into a throbbing baseline that sounds like nothing so much as something big and scary coming to eat your face.
Nintendo Hard: Especially if you don't know how to best develop your party.
At the end of the Nasty Dungeon, the same fairy who warned you not to enter asked you what you thought of it. If you say it wasn't so nasty, she teleports you back smack-dab in its middle to comb your way out again.
You can just teleport yourself out if you answer the wrong way with the MAGI that you get right by the entrance.
That One Attack: The final boss' Smasher ignores defense and distributes damage randomly among your party.
Apollo's Flare, dealing almost 1000 damage to everyone it hits (instant death to one if you didn't raise its Spirit (Mana) high enough). Especially in the remake.
That One Boss: Venus and Apollo tend to be stumbling blocks for many players. Though Apollo will die on his own, so you can just sit back and heal constantly.
It's actually advisable for the whole party to die at least once during gameplay, because if you haven't died at all up until you reached Odin, then the fight with him will be much harder.
Odin as well. The OdinCrows with him can throw unpreventable damage on your entire party. Odin himself has Gungnir that can one-shot anyone in your party (unless they have 999+ HP and good armor — or O-Weapon). (Hint: equip one of your members with the Aegis magi and use it each round).
That One Attack: The Defense System's Double Starburst attack, which tends to hit for about 550 HP of damage and which it will use on every turn past a certain point. If your healer isn't the first to attack by the time it begins using this move, you're dead.