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.
Breather Boss: Gimle in The Millennium Girl. It's insanely bulky, but it isn't a threat at all when you consider that you just got past the Iwaoropenelep. Doubly so if you have Peace Ballad, as it removes any need for even TP restoring items.
Demonic Spiders: Petaloids their variants in Etrian Odyssey loved nothing more than to chain-sleep your entire party, thus allowing the other enemies to unleash a world of hurt on you.
They appear in Legends of the Titan as well, although thankfully they don't always attack your entire party.
The Bloodant FOEs in Etrian Odyssey: the first time you encounter them on floor 12 they don't hit very hard but their Hp is significantly high and they regenerate indefinitely, they can also summon deathants AND bloodants on every turn making it even harder to get past them. They just keep fighting you until your entire party dies from exhaustion. Also, four of them pop right out of nowhere when you fight the Royalant Boss. They're each in a corner of the room, and there is no way you can beat the Royalant before they start moving on. Enjoy a main dish of Royalant, a side order of Bloodants with a generous helping of Deathants with your-ass-getting-kicked sauce. Thankfully, they are much easier in the remake.
In general, there is at least one type of enemy on each stratum in every game that could qualify as one. In particular, the final strata have plenty of enemies that can get very dangerous unless handled right. The Drowned City has the Longicorn Beta on the final floor that can summon multiple enemies at any time, and then throws them for high damage to the entire party. It's also highly durable, which makes it difficult to shut it down with statuses long enough or kill fast enough to stop it from throwing enemies.
In The Drowned City, there are also monsters that are otherwise weak, but they qualify as Demonic Spiders because if you don't kill them fast enough, they will summon or transform to FOE-type monsters. A really nasty surprise if you don't expect it.
And in Legends of the Titan, the Hollows are meant to be a crash course in utilizing Bindings, and it shows. Even the weakest Hollow type has evasion that's through the roof, so unless you have a Sniper or Arcanist in your party, just hitting one can be a Luck-Based Mission.
Also from Legends of the Titan, there are enemies in the final dungeon that can petrify your entire party with one move. While they only target one member of your party until damage is dealt to it, they're occasionally paired with an enemy (a Hollow, no less) that can deal one point of damage (which is enough to make it change from targeting one member to your whole party) to get a power boost.
Designated Villain: Visil in Etrian Odyssey, who as it turns out is trying to save the world.
Arthur from Millennium Girl receives a lot of attention from fans due to being rather comical.
Esoteric Happy Ending: The first game. Hooray! You've defeated Visil and uncovered the truth! Too bad it involved killing the guy who probably saved the world and likely plunging Etria into an economic disaster.
Game Breaker: In a game where most classes had roughly 300 HP at max level, Etrian Odyssey's "Immunize" was the difference between most enemies hitting for 250 and for 25. Because Atlus has a personal vendetta against anyone who ever succeeds in any of their games, rather than rebalancing it, they just removed Immunize entirely from Heroes of Lagaard. Oh, and no more Healing and Relaxing for Troubadours, either. Have fun!
Which didn't help much, because they also took the opportunity to boost some unused skills, and did a bit too much. In Heroes of Lagaard, the Hexer skill Revenge was boosted to deal 255% of the damage the Hexer had taken for only 19 TP once mastered, which means a Hexer with items to boost max HP, 1 HP left, and enough AGI to move first can be devastating.
To elaborate: Revenge deals set damage depending on how much HP a Hexer has lost, ignoring enemy's defense, resistance, immunity etc; If a hexer can deal 1000 damage to a mook, they can deal that much damage to a Boss. There are two reasons why Revenge is broken in Heroes of Lagaard but not in Etrian Odyssey: First, Revenge is stronger in EO2, and second, most importantly, FOEs and bosses have significantly lower HP in EO2: EO1 bosses can have upwards to 5,000-10,000 HP and even beyond, but in EO2, highest HP among non-Final Boss or Bonus Boss is around 3000-4000, but said bosses have such massive defenses that Other classes will have to be lucky or maxed out to deal a couple hundreds of damage to a boss. With proper equipments and setup, EO2 Hexers can dish out between 1500-2000 damage per Revenge. This entire thing is why Revenge is a Game Breaker: A Boss Fight using other classes can last 10 turns or even longer, while Hexers can happily wipe out a boss in probably two hits.]]
While not as broken, the Hexer, with Evil Eye and Suicide, can command the enemy to attack itself instead of the party, allowing the player to attack an enemy without worry of being hit. There's also Betrayal, which forces all terrified enemies to attack each other. Adding in Relapse, it's possible to cripple an enemy for the rest of the battle.
In The Millennium Girl, Evil Eye was buffed. Instead of just targeting one enemy at a time, it targets a row of enemies. Curses increasing ailment chance instead of only allowing the caster to learn new hexes makes it more reliable as well, making Hexers even more powerful than they were before.
Also, the Whip It Good Dark Hunter's Climax, once mastered, kills any enemy vulnerable to instant death with less than 55% of its HP (previously, it only had a 20% chance of working, even when mastered). Most bosses are immune to Climax, but FOE's aren't; having Climax mastered essentially halves the duration of any FOE encounter.
A War Magus' Cursecut can drain TP from an enemy that's been cursed (best done with a Hexer, which is a Game Breaker in and of itself). War Magi can also transfer their TP to other characters by the use of their 'Transfer' skill. With this combo, you have potentially limitless TP. A shame that this is pretty much all that a War Magus is good for...
In addition, the Hexer's Torpor immensely aids the War Magus' Sleep Cut, allowing the party to deal pretty massive damage while the enemies are asleep.
The Dark Hunter's Dominate. It's guaranteed to go last, but it's also guaranteed to bind any enemy, including bosses, preventing them to use their skills (beware: some exceptions exist). And bosses generally will try to use skills even when bound, wasting their turns. Couple that with Dark Hunter's other skill, Ecstasy, which deals massive damage to a completely bound enemy, and you can often win quite easily. Also, using a Survivalist's 1st Turn skill allows the Dark Hunter to use Dominate before anyone else moves.
Heroes of High Lagaard's Force skills in general are fairly severe Game Breakers. The Gunner's guarantees that you'll stun an enemy for that turn. The Protector's negates all damage and status effects for one turn. The Dark Hunter's fully binds an enemy. At the end of the game, it's possible to use them every turn by dedicating two characters to feeding the Force skill users items that raise the Force gauge. It's very costly, though, and requires you to gather materials, so it isn't suited for spamming.
The Dancer's bottommost dancing skills in Legends of the Titan. They're all passive, and give an increased dodge chance, the ability to hit multiple times in a single attack, and the chance to stun an enemy upon connecting with a normal attack. It only takes three skill points to unlock the second skill, compared to the sixteen (at least eight of which are dumped in a useless skill regardless of what weapon you use) it took to unlock Swashbuckling (which is the same skill) in The Drowned City. Dancers are also able to use a different skill to make allies in the same line follow up on their attacks, meaning it's possible to have your entire front line attack multiple times per turn.
The final class in Legends of the Titan count as well. They're restricted to using only one type of weapon (they can't equip a subweapon alongside their class-specific weapon) but are capable of dealing thousands of damage while unboosted. To offset the massive amount of damage being dealt, they're given a few cooldown turns until they're allowed to use it again. However, they can use two skills to cool down their weapon (which deals damage as well, thus preventing them from being useless) and then use an attack that charges up their next turn's attack power (which more than doubles their power even without the skill maxed), which just so happens to be the same turn they're allowed to use the powerful attack. If an enemy somehow manages to survive a few of those attacks, there's even a skill that removes the cooldown time for three turns and immediately cools down their weapon after a certain amount of uses were performed in a single battle, allowing the character to deal massive damage without any drawback for three turns.
Once a Nightseeker in Legends of the Titan hits high enough level to be able to use Venom Throw, maxing it out allows them to inflict a poison status that does a fixed 700 damage every turn for several turns, enough to make a large dent in even most bosses' HP: combine this with Spread Throw and Auto-Spread which allows them to hit every enemy with the next 2 throw skills and a chance to use the said skill for free at the beginning of the battle, and they can take out most random encounters in a single turn with very little TP used. If you then give them Arcanist as a subclass, you can not only use one of their passive abilities to increase the chance of landing the poison status on even strongest enemies, but also gain back any TP you spend to use the move if the poison sticks on at least a single enemy thanks to their TP Return.
Hexers break the game again in The Millennium Girl with Stoning Curse, an extremely reliable petrification spell (which instantly kills the target) against normal enemies. Unless there's a massive group of enemies that require Evil Eye, Stoning Curse is all a Hexer needs to use outside of F.O.E. fights and boss battles. The best part? It doesn't even cost that much TP.
Again in The Millennium Girl, there's the Gunner skill Action Boost. It allows you to take any action again (and again, if you level it up enough), but charges you TP for each consecutive use for your next turn. The game breaking part is that, provided the skill that triggers it is increased by Boost, all attacks in that chain are boosted. This allows you to fire off multiple powerful skills such as Ricochet under the influence of Boost without any drawback, provided you have enough TP to use it multiple times.
Good Bad Bugs: While the Monk's Fist skills require you to fight unarmed, for some odd reason, the second slot counts as a Weapon... if your second Item Slot/First Armor Slot is blank, a Monk can use his/her fist skills with a Mace, even though the Monk does NOT have Second Sword (a Shogun class skill)... whoops.
You can skip Fenrir in the first game by exploiting the fact that FOEs maintain aggro even with a wall between you to lure him into a corridor so you can run past him. While you can not go far on the 6th floor without going back and killing him (as you need to report Fenrir's death to do so), it does allow you to warp to town, save, and fight stronger monsters for better gear and get a free turn on Fenrir by attacking him from behind, as well as open some new quests. This is all very helpful, as Fenrir is an Early Bird Boss.
There are two notable and extremely useful glitches in The Millennium Girl; one allows infinite replication of any item that can be used in battle, while the other lets you pass down a large number of skill slots from a grimoire in synthesis without actually consuming it.
Hell Is That Noise: In Legends of the Titan, a simple "ding!" sound plays whenever a FOE takes notice of you and starts actively chasing you down. A simple little thing, but you'll quickly learn to dread that sound, especially if you're a veteran to the series.
Moe: Present throughout the series, but the female Yggdroids from The Drowned City take the cake. The males look like you'd expect humanoid robots to look, but the females really had moe shoehorned onto them. They're tiny, have human faces and look like they would break if you looked at them too hard. (also, why would robots need to wear glasses?)
Abigail from Heroes of Lagaard and Missy from The Drowned City also count.
Moral Event Horizon: In The Drowned City, not only does Olympia betray the party after pretending to guide them and leave them to die against seemingly impossible odds multiple times, its heavily implied that she similarly led countless other explorers to their deaths, possibly even murdering them herself, thanks to her orders to keep explorers from discovering the Deep City at any cost.
Player Punch: Early in The Drowned City, a quest becomes available to locate a camping-obsessed guild that has been missing in the labyrinth for several days. Following the clues they leave from campsite to campsite, the last you stumble on is described as a scene of carnage (blood everywhere, tattered and broken equipment and whatnot); naturally, you believe they were all killed by monsters. Subverted when you go to report the quest at the pub, as Missy almost flat-out tells you that the guild members are freakin' fine, and that all the blood at the campsite was from the monsters that interrupted their beauty sleep.
Played jarringly straight later: If you agreed to help Hypatia and Agata enter the second stratum, then you eventually find them there, and Agata presses you to tell him the location of a nest of Sea Wanderers you found earlier while Hypatia begs you not to, as she already had a traumatizing encounter with them before. You just decide whether to tell him or not and think nothing of it when he runs off to find them. Then you decide to follow him, and go to the room and find one of the two fucking dead while the other is cradling their body. There is no way to avoid this happening, as saying "Yes" or "No" only determines which dies (saying "Yes" means Agata dies, while "No" kills Hypatia). There is no way to back out of this, except for refusing to help them enter the second stratum in the first place. Then you literally never see them again, and you're free to pretendthey gave up and went home peacefully if you'd like. A bit anticlimactic, but hey.
Replacement Scrappy: When early info about The Drowned City was released, and fans learned the original classes wouldn't return, Gladiators were blasted as being generic and vastly inferior 'replacements' for the Dark Hunter class, despite players not knowing anything about it beyond the physical appearance of one representative. Thankfully, this reaction died down over time.
Even funnier when you finally found out that Gladiator is supposed to replace Landsknecht instead. note The Princess gets the Landsknecht's ability to equip heavy armor, while the Gladiator is a straight combatant; while their armor choices emulate Dark Hunters, they generally lack the ability to dole out status effects unless they invest in Stun Attack. But in turn, when compared to Landsknechts, both classes can specialize in two weapons: Sword and Axe/Hammer, and their sword skills rely on hitting as many enemies as possible, while their other weapon skill rely on hitting one enemy as hard as possible.
That One Attack: The Yggdrasil Core's Cell Membrane in The Millennium Girl. It prevents any and all damage for a turn and releases a painful party-wide counter in return for getting hit. If you just so happen to have a character that has the Highlander skill that may cause an attack upon being dealt damage, you may go down from a single attack triggering a chain. While it's predictable with a guide that explains the boss' patterns, it will break the pattern if you apply enough buffs, meaning you can die on any turn you choose to attack.
To explain, you need to beat every single F.O.E. on the floor to actually kill it on a floor completely filled with them. While it's possible to fight and beat Iwaopeln before nearby F.O.E.s reach you, it just instantly respawns if there's so much as a single one left. He also has a really powerful attack that will kill at least two of your party members if you don't have Immunize.
Fortunately, this is not required in the remake (as they disappear as soon as the bird is defeated), although it can use the F.O.Es to assist it in battle, as well as respawning them if it sees you.
They Changed It, Now It Sucks: When the first preview screens of The Drowned City were released, some fans complained about the new seafaring setting and the new roster of character classes, bemoaning the loss of the classes from the first two games.
Viewer Gender Confusion: And how. All classes have four character portraits to choose from. Two are male, and two are female. This is highly important information for the Survivalist, Troubadour, and Hoplite classes in particular, which have some portraits that can only be identified via process of elimination.
Woolseyism: Atlus changed the names of the character classes during translation; Landsknechts were originally Swordman, Protectors were Paladins, Survivalists were Rangers, and so on. This may have been done to give the game a more original flair and help it stand out. The Hexer class was originally called 'Curse Maker'; the War Magus was originally a 'Doctor Mage'. A newer example from The Drowned City would be changing 'Beast King' to 'Wildling', probably because (like most classes) you can make a female version, and the original game didn't change the class name to 'Beast Queen' like it did for Prince/Princess. Some classes got their names changed for no reason at all but to make them begin with different letters. Many of the changed names don't fit as well as the original games: Warrior became Gladiator despite not actually being a gladiator, and Andro became Yggdroid. Many of the changes in the first and second game were necessary because of the maximum character length, though the third game increased the limit. Some changes were made even though the original names would have fit the character limit.
It was (probably) also to avoid classes having the same first letter in their name as to make it easier for the item shop menu to characterize them. It would have been a little difficult to tell the difference between Princess and Pirate, or Buccaneer and Beast King.