In the NES version, in an early fight at Dragon's Peak, Bahamut appears and more or less eliminates the party without so much as a qualm. It's entirely possible to grind one's way up to a point where he can be defeated, though. If the player does so, the plotline will still progress — but he won't appear again later, making it much more difficult to obtain his summon.
In the remake, the conversations you can have with guests are upgraded to full-on cutscenes. The foresight is twofold - while most interiors use the same general stage, certain places such as the magic shop will have their own stage and certain conversations (most easily seen in Tokkul or the Village of the Ancients when traveling with Desch) will actually be extended depending on whether certain NP Cs are talked to or not. This is especially noticeable with Tokkul as part of the cutscene will be skipped depending on whether or not a certain NPC is talked to before the one that adds the end of the cutscene.
There is a part of the game in which which Golbez and Fusoya fight Zemus, and afterwards, Zeromus. In most versions of the game, this is simply an AI fight and Golbez cannot be used outside of it (as he lacks the proper animations, stats and whatnot). However, hacking him into the party in the 3D remake shows that he is fully functional in that version. He even has stats, a job (Lunarian), specific sets of armor/weapons he can equip, the ability to use Black Magic, a victory animation and a menu portrait. The only thing he is lacking is an overworld model, as the game uses his younger self (Theodor) instead.
Krile gets knocked out by a psychic migraine from Ghido. If you return to Bal before completing the Barrier Tower, she'll respond with the same line about Ghido calling. Since Ghido is halfway around the world and requires a new vehicle to reach him, it's pretty common to move right on to the Forest of Moore after you finally get to him. But if you do go all the way back to Xezat's fleet, grab the wind drake, and fly all the way back to Bal, Krile will say that her headache has cleared right up (although Galuf tells her to stay in bed anyway).
During Galuf's Last Stand against Exdeath, it's normal for the player to spend most of the fight at 0 HP but if you use the Blue Magic Self-Destruct, the fight ends instantly no matter how much damage you've done to Exdeath.
The boss Omniscient normally exclusively uses powerful spells, but is vulnerable to the Berserk and Silence statuses. Sounds simple, right? Unfortunately, Omniscient also has one of the highest attack stats in the game, capable of one-shotting party members, and will put it to use if his magic isn't available. While there are still ways to deal with this, it's a clever counter to what could've been a cheesy tactic.
There are a few cutscenes (such as when you reach Ramuh and Esper Terra) where the dialogue for your party members is not character-specific. Every party member will say the same thing, no matter who's in what position. But if your party consists of only Wild Child Gau and no one else, the dialogue changes to properly reflect Gau's Hulk Speak in these cutscenes.
Every playable character has the same range of movement and expression sprites laughing, raising their arms, getting hurt, etc. The foresight comes from the fact that even Guest Star Party Members like General Leo, Bannon, the Phantom Train ghosts, and Locke's merchant disguise have these animations as well, even though they wouldn't be seen in a normal playthrough. So if you hack or glitch them into the party, you'll still see their animations play. Even Esper Terra and the Imp (which are normally impossible to have as party leaders) have a full range of animations for this purpose, just in case.
Normally, party members who aren't in the active party loiter around the airship deck, whether it be the Blackjack or the Falcon. If you somehow hack Umaro or Gogo into your party in the World of Balance (which is normally impossible), they still have places assigned to them in the Blackjack for them to appear when they're not in the party. The same goes for Shadow, who normally isn't ever seen on the airship in the World of Balance.
If you try to bring Shadow to Narshe, where you would have the option of changing your party, he leaves. He also leaves if you hire him before going to the opera house, in case you thought to try and get him to come to Vector.note Though this does lead to a Game-Breaking Bug, where trying to enter Narshe with just Shadow will still have him leave, leaving you with a party consisting of no one. The only thing you can do at that point is open the menu, and closing it causes the game to crash. However, the only legitimate way to have such a party at this point in the game is to form a party of only Gau, hire Shadow, cross the entire world over to the Veldt without letting Shadow run on his own, have Gau leap away, and return to Narshe with Shadow alone, making this Unwinnable by Insanity.
If you get to the Final Boss without re-recruiting Terra, there will be a scene immediately before the part where you select the party lineup for the final battle. Terra leaves the orphans of Mobliz behind, transforms into an Esper, and flies toward Kefka's Tower.
There are a few instances where a character that the player couldn't possibly use legitimately at certain points in the story still has dialogue coded in to prevent the game from crashing.
Tifa and Cid get special dialogue for generic sidequests if you do them when they're leading the party. Some of it changes the plot slightly - for instance, when meeting Yuffie, where Cloud tells Yuffie he doesn't care, Tifa will try to reason with her. They even have dialogue for scenes that it's not possible to legitimately access with Cid or Tifa as leader; like the buggy breaking, or leaving the Chocobo outside the mines if you haven't been to Kalm yet.
Normally, Don Corneo's reappearance is a time for Cloud (and/or Tifa and Aerith) to face an old enemy again, and the sequence is written as such. But if you bring a party to the Wutai Theft quest that doesn't contain anyone who had previously encountered Don Corneo (e.g. with Cid as leader), the scene changes to make it clear that neither Cid nor the Don have any idea who the other character is or why they're messing with each other.
In areas where you can talk to your party members, dialogue for Aerith, Kid Cloud and Sephiroth is still added at those points, in case players hacked the game to add those characters to their party. This is incidentally what fueled the rumors that there was a way to revive Aerith, as players noted that she had new dialogue if she was hacked back into the party after her death.
The Post-Final Boss fight between Cloud and Sephiroth is supposed to be ended in one move by unleashing Cloud's final Limit Break, Omnislash. But if you just stand there and do nothing, Sephiroth will eventually attack Cloud. This attack is gravity-based, so it can't kill Cloud. Even then, Cloud will automatically counter-attack. Sephiroth also has only one HP, so Cloud's attack will always be strong enough to kill Sephiroth. In fact, searching the game's code shows just how much foresight went to this battle. Attack Sephiroth and do no damage? Attempt an attack and miss? Attack and heal him? He'll still go down.
When the player uses the Scan spell, the player can rotate the target in all directions to examine it. If Selphie is scanned, the player can only rotate her 360 degrees, preventing the player from looking up her skirt.
If all protagonists are together in Balamb Garden, the ones not in the party will be found wandering around.
Cheats to make the party invincible in the final battle will prompt Ultimecia to cast Dispel and remove the effects.
During the "chasing Princess Garnet" segment in Alexandria, Garnet will run down a set of stairs while Ruby is complaining to Zidane. If the player had Zidane block the stairs while talking to Ruby, Zidane will jump out of Garnet's way as she runs down the stairs.
Quina is an Optional Party Member during the first disc. If you recruit him/her, the game will acknowledge that s/he is with the party during Disc 2, and s/he is Put on a Bus via a Running Gag where Quina is almost always the one left behind. Returning to Qu Marsh on Disc 2 will have Quina greeting Zidane&Co and Zidane asking how s/he escaped Cleyra. If the player did not recruit Quina in Disc 1, then the scene that plays when they just meet Quina for the first time plays.
Quina has a minigame that sees him/her catching frogs in order to receive gifts from their mentor. If Quina catches all the frogs, it will take some time before some start to spawn again. However if you leave a female frog and a male frog behind, the frogs will spawn much more quickly. You can even see them as little tadpoles before they become frogs.
The Soft item cures the Petrify status. Using a Soft on stone enemies will instantly kill them. This and Final Fantasy V are the only games where it is possible to do this. This can even be used to defeat Soulcage (normally a fairly challenging boss) instantly. Hint: It's an Undead enemy just like the zombies and Dracoliches in the area leading to it, so healing and especially revival items / magic are its fatal weakness.
Garnet/Dagger's Summon option includes a bit of Gameplay and Story Integration. At the beginning of the game, all the Summons have MP costs that are far too high to use. A later plot event unlocks her latent summoning powers, at which point the MP cost for the Summons drops enough to be viable. If you level grind excessively until you have enough MP for the summons, you still can't use them, because Garnet's power hasn't been awakened yet.
If Eiko summons one of her Eidolons during the fight with the Hilgigas, Dagger will acknowledge it.
At one point during the game, Quina can be found eating lots pickles from a shopkeeper without paying, much to said shopkeeper's dismay. If Zidane comes to "rescue" Quina during around this time, he pays the money Quina owes her just so that she's satisfied, which happens to be 100 gil. If, for whatever reason, Zidane does not have that many funds, he'll put the tab on Baku instead, leading to an Active Time Event Easter Egg involving said lady asking him for the money.
The "Friendly Enemy" side quest is supposed to not only allow Ozma to be hit by physical attacks, but also make it weak to Shadow instead of being healed by it. Beating Ozma before completing this side quest will cause the last "Friendly Enemy" to comment about it.
While there's no reason for the party to return to Ispen's Castle after Terra is destroyed and Mist now covers all of Gaia, the mist-free exterior of the castle has been replaced with a mist covered version. And while various places are sealed off from Disc Four onwards in order to free up memory, there's an in-story reason for it too; the roots of the Iifa tree have gone wild, sprouting up from the earth and physically blocking certain areas.
Additionally after defeating Taharka in Ipsen's Castle, a cutscene will show one character nearly falling through a trapdoor. Who this happens to depends on who's in the party, similar to the hierarchy system. First Garnet, then Eiko, then Vivi, then Quina.
There are various NPCs scattered throughout the world that will live out their own stories during the course of the game. Backtracking and talking to them after certain story events will see their dialogue changing to focus on what has just happened. This is especially noticeable if you talk to the citizens of Alexandria or Lindblum after they've been attacked. The girl that runs the item store in the first town? She and her mother have a small story arc that can end up with Zidane telling them they should take a vacation together; on returning to the store, you'll find that they did just that. The pickle seller in Lindblum will mention she's expecting grandchildren, will lament the fact that she cannot see them as she was blinded in Brahne's attack, but then ultimately decides to be grateful for the little things in life.
While it's a big Guide Dang It!, it's possible to obtain the aeon Anima just after defeating Sin for the final time. The cutscene that plays once you get it reveals that Anima was created from Seymour's mother becoming a fayth. Summoning Anima against Seymour Omnis (the only battle against any of Seymour's forms in which you might have Anima) has Seymour comment "You would oppose me as well? So be it!"
A plot point is that Rikku has a Fear of Thunder due to Brother accidentally hitting her with a Thunder spell. If she is hit with a Thunder-element attack, you'll hear her scream, whereas everyone else will just say "Ow" or something else.
Backtracking is purely optional throughout most of the game, but if you do so starting after Operation Mi'ihen, you'll get a lot of extra content. For example:
NPC dialog is updated as events happen in the game. For example backtracking after Operation Mi'ihen, people will be talking about what happened and the fallout from it. Once Seymour proposes to marry Yuna, that will then become a major topic among the NPCs.
After Operation Mi'ihen, one Crusader will say that their losses were so severe they won't be able to perform their normal function of patrolling the roads and protecting travelers. He'll also say that he heard that the Yevon warrior monks and the Guado will be picking up the slack. Sure enough, if you go backwards you'll find the warrior monks and Guado patrolling in place of the Crusaders.
NPCs go on with their lives as you go about your quest. Repeatedly backtracking and talking to everyone will sometimes result in you seeing the lives and situations of no name NPCs changing multiple times.
At one point, there's a scene in The Farplane where both Trommel and Lulu are there, each looking for someone significant to them who is presumed dead. If you haven't defeated and sent the souls of those two people yet, there will be a different scene with different dialog.
If the player doesn't stop Garik from attacking the Guado, you'll see the Ronso's attack on the Guado via the Comm Spheres.
In Final Fantasy XIII-2, the location of Augusta Tower AF 200 can only be traversed for a short while, until Serah and Noel come across a gap and a keypad that requires a certain keycard to have the necessary bridge appear and allow them to continue. The keycard in question is actually stuck in Augusta Tower AF 300, a previously unlocked location that can be mistaken as just one of the few optional locations. If the player happens to have gone there previously and obtained the keycard, Serah will mention that they already have the keycard and they can instantly proceed.
In Final Fantasy XIV, completing certain quests in different orders will yield slightly different NPC dialog to reflect this. For example, if you complete "The Adventure of the Fainting Goldsmith" before "Manderville Men", then when you meet Godbert Manderville a second time, he will remember that you gave him a fantastic massage and will thus recognize you as "s/he of strong yet supple fingers". Things like your rank in your Grand Company will also get referenced depending on who you are currently talking to, and many NPCs will have their dialogue to adjust for current game events, even if the NPCs in question is not super important.
Guest party members will also join in on photo ops. Even Aranea, who you really shouldn't have the chance to do so during the chapter where she joins up. Likely it's because she will occasionally join you in battle after chapter 9, and players can end up triggering photo ops during these times. Becomes quite noticeable when using the exploit to permanently add her to the party.
Specific resting locations may have others join you as well: sleeping in Hammerhead's caravan may occasionally have Cindy hang out with the guys, or the house in Cape Caem (Ch. 6 onward) can have Iris and Talcott similarly hang out.
The summoning animations actually change depending on the player's location and circumstances, such as whether they're above or below ground, what the enemy is (Titan will actually punch the Adamantoise first if you summon him in that fight), etc.
Your thunder spells grow stronger when it's raining, and can electrocute you if you're standing in water when you use them. Blizzaga spells can also freeze water solid, and again, if you're standing in it when you use them, you and your enemies will get stuck until it thaws out.
The ending cutscene, where in the afterlife, Noctis shows Luna a picture, chosen by the player beforehand. No matter what it is, Luna will smile before the two share a kiss, but if the picture's from the party's first visit to Altissa - including, particularly, a pic of Luna's wedding dress - Luna will also shed a tear.