In EverQuest, the Mana Burn ability given to wizards meant that you could take down any boss in the game simply by assembling enough wizards who had gained that ability, and all they had to do was be coordinated enough to all cast at roughly the same time. This was particularly game-breaking with respect to bosses whose difficulty was regeneration- or threat-based. Teams of Mana Burn wizards used to prowl for fun and profit, even working for hire. Point and shoot, effective one-hit-kill boss fights. It didn't take very long at all before the nerf bat struck, and all designated raid bosses in the game, no matter what level, were given a 1 minute invulnerability to both Manaburn and the shadowknight's Harm Touch (both of which completely ignored all forms of damage mitigation and resistance, save for Divine Intervention.)
In EverQuest 2, one of the level 70 top raid mobs dropped a fighter item. A leather pair of boots with a proc on them that had a chance to make the wearer immune to damage for one hit. On their own they were good but not overpowered, but then came along AA's which boosted proc chance and procs that also boosted proc chance and they became the best boots in the game to raid tank on, selling for hundreds of dollars. Then came the nerfbat, changing the proc to only be once every 10 seconds, stopping infinite stoneskins.
When first introduced EVE Online's Doomsday Devices (a massively damaging AoE attacknote dealing enough damage to instantly destroy any ship smaller than a Battleship, and even Battleships will fall if not specifically tanked against them) could be fired remotely, and imposed no penalties on the ship which fired them. This often resulted in battles being decided simply by which faction activated theirs first. One Obvious Rule Patch later the ability to fire remotely was removed, and they now disabled the ships jump-drive for 10 minutes when fired, turning them into more balanced last-resort weapons.
Doomsdays are becoming a Game Breaker again for a different reason: one doomsday is survivable, a second is not. The Double Doomsday is now a common technique for obliterating sub-capital fleets. In one show of force, Goonswarm and allies set off 16 Doomsdays, enough to obliterate capital fleets as well.
And now doomsdays have been changed again: to a single-target One-Hit Kill.
Speed was arguably even worse than Doomsdays. If a ship is fast enough it can outrun all incoming damage (short of AoE effects). Nanofiber modules and Polycarbon rigs which boosted speed and maneuverability were for a time de rigueur in small-gang combat. Eventually CCP realized that when people's advice when fitting any cruiser was 'nano it' this might have broken the game somewhat. It remains possible to make your ship invulnerable by orbiting fast and close but only for frigates and the tactic can be countered very simply by not flying solo as your friends can easily pull range and nail the bastard.
Speed was in fact even worse back in the day when Battleships would be nano'd to invulnerability.
With that said, EVE has always had balance issues be it Heatsinks, Artillery, Nano, Titans, Capitals or whatever. The devs tend to think up cool things first and the ramifications (much) later.
Motherships/Supercarriers are straying into this territory now. Groups of them are virtually unkillable due to their remote-repair abilities, and they can still deal massive damage while doing this due to their fighter-bombers, which will automatically follow a target that warps off. They also cost significantly less than Titans do and use skills any competent carrier pilot likely has already, making them much more common. Doubly so in low-security space where they're even harder to hold down and Titans can't fire Doomsdays at them (or anything else).
Who can forget the brief reign of the Falcon, which was capable of using its ECM to reliably jam multiple battleships from extreme range while using its cloaking ability to hide until needed and its range to stay out of trouble? Mitigated with a patch that severely reduced the range of ECM-dedicated ships to a distance where they're likely to take some returning fire.
In 2012, EVE introduced a new "ancillary" shield booster, powered by cap-booster batteries. It quickly proved immensely popular because a) it doesn't use a ship's capacitor energy, leaving that available for things like weapons, afterburners, tackle modules, etc.; and b) it's insanely overpowered for its price (as in: a medium ancillary booster runs one or two million ISK, and to find a standard booster of equivalent power, you have to look for faction or deadspace modules which each cost as much as a battleship). The only drawback is that when it runs out of battery charges, it takes a full minute to reload ... which is why enterprising frigate pilots mount two of them.
Ragnarok Online had the silver shotels, swords with 50% critical rates, when an assassin can wield them, they can practically hit anything but their aspd will be shot to hell. Not a true gamebreaker but very problematic.
Gravity have recognised and dealt with Gamebreakers in the past, too. The most notable of them was the Dancer+Bard combination skill, Ragnarok, which currently languishes fully implemented, but disabled, as Gravity still can't work out how to stop it being broken without making it pointless. Basically the performance skill requires one bard, and one dancer, both built around very specific skill trees in order to attain the Ragnarok skill itself. Once activated, Ragnarok initiates one randomly selected type 1 (highest category) spell, from any type 1 spell in the game, every 2 seconds over the course of one minute, be it a healing spell or offensive spell. The performers are paralysed for the duration of the spell, but it will not end if they are killed, and it hits both enemies and allies alike.
Going clear back to Pre-CU! Of the three Damageable stats, only Mind could not be buffed by doctors. The relevant entertainer buffs were a small fraction of what the doc buffs were doing for the other six stats, and even with food and drinks thrown in you could only get sorta kinda close, and for a shorter duration. To top it off, mind was the only stat that couldn't be healed by the otherwise overpowered (like the doc buffs!) standard stimpacks. Now throw in the fact that each combat class could deliberately focus their attacks on a specific one of the three pools. The classes able to target mind generally ended up overpowered in PvP. Similarly for weapons with mind-affecting Damage over Time bonuses, particularly fire. For round two, throw in that the most common type of armor in the game could be, and was, readily made to have protection against all types of damage EXCEPT stun, which most profs couldn't use. Eventually Composite armor with protection against both kinetic and stun did emerge, but the stun protection was always the weakest and it tended to lose some on the other protections to boot... Now, wrap it up by having only ONE combination of profs that can use a stun weapon (Geonosian Sonic Pistol) with a mind-pool attack (Eye Shot). Care to guess how common Bounty Hunter/Pistoleers were? (The fact that people were pushing the crafting system as far as it would go to get ANY stun protection is a hint)
An older example: Pistoleers had two specials called Knockdown Shot and Dizzy Shot. Many a player who did not know there was a pistoleer in their midst often found themselves attempting Knockdown Recovery, to no avail. The only message they would get on screen was "You cannot Knockdown Recovery while Dizzy."
Commandos are easily the most broken profession in the game. It is possible, with endgame rewards to stack Block Chance over 60% and Block Value to over 1300, giving Commandos the highest passive chance to block an attack (and also the highest amount of damage to passively block) in the game. Add to this an ability that can reduce incoming damage by 60% (75% if you spend more points in that expertise line) and you have the preferred tank (which is what a Light Side template Jedi is supposed to be). This would all be fine and dandy if Commandos werent' also the ''hardest hitting AoE/DoT sustained damage dealers in the game''. Short of ganking a Commando (who is usually overbuffed for maximum WTF factor) in a group of at least FOUR OTHER PLAYERS, a Commando can completely wipe a player with next to no skill involved.
There's also the Spy, who can deal an INSANE amount of spike damage, starting with an attack FROM STEALTH (and if the spy is stacking Camouflage, you can bet your sweet ass you'll never see it coming) that not only results in a Critical Hit for upwards of 4000 damage, ALSO brands you with a debuff that lowers your healing potency, slaps you with a DoT effect (Burn or Bleed, pick your poison), and snares your character. Add that to the fact that while wearing endgame rewards they cannot be snared themselves and that they have at least three stealth specials that they can use to escape combat and usually remain stealthed indefinitely, and you have the makings of a Game Breaker. Not even taken into account is the fact that it's nearly impossible to get your Detect Camouflage modifier high enough to pop a spy from stealth without using up modifier slots needed for FAR more important mods... unless you're a spy with points spent in the Reveal expertise lines. Pretty funny that it takes a spy to pop another one from stealth.
Early on in City of Heroes, there was no limit to the amount of spawns a player could herd, or a limit on how many could be affected by an area of effect attack. It was possible for a single player with enough defense and/or regeneration to herd every enemy on an entire map to one location, and either use an AoE attack themselves (or have a teammate do it) to wipe them all out in one shot, getting massive experience points with practically no risk to anyone else on the team. And if the mission required something other than merely taking out enemies to finish it, they could quit and restart the mission and farm it all over again. This was later changed by stopping mobs from mindlessly following the herder (if they got too far away, they'd stop chasing), limiting how many critters could be angry at a given player (No more than the largest amount of critters in one spawn—others would break off and attack other targets, or just disengage), and putting a limit on how many critters could be affected by a single attack.
This was made worse by a few other game breakers, especially the Tanker Invincibility power. It gave a bit of defense for every enemy nearby... except it ticked faster than the defense faded, and had too high of a defense value in the first place, so the resulting Tanker became nearly impossible to hit with just a bunch of weak enemies nearby. While not completely impossible to kill, it was pretty close. Thankfully fixed.
Scrapper Regeneration used to allow the old version of Moment of Glory to recharge faster than the effect expired, and each activation completely refilled the player's health and endurance bar. To be defeated, enemies would have to do hundreds of times a normal player's lifebar in damage within four minutes. Outside of that, Integration and Instant Healing made the scrapper nearly impossible for many players to defeat in PvP.
There was a time during the City of Heroes beta where resistances (which reduces damage by X percent) were allowed to reach 99%. This occurred before the damage was rounded off which generally reduced it even further or even made it non-existent. More than one tank ended up dumpster diving with obscenely large amounts of monsters just to see where they could lure monsters or how many they could attract. One beta tester in particular ended up tanking an entire zone of monsters many levels above his own all by himself.
Buff, debuff, and status effects in City of Heroes are much stronger than is typical for an MMO, and most buffs don't affect the caster. A single support character on his own is typically fairly weak; a team consisting of eight of them and nothing else is stupidly overpowered. People unfamiliar with the game often suggest "ultimate teams" of seven support characters and a tank, the standard response to which is to inquire as to why the support types should carry around the dead weight.
A popular example is a team of eight Empathy Defenders: a Empath's minor defense buffs stacked eight times gives more defense than a tank, the rapid HP recovery of an Empath stacks so far that if it can't kill you in twelve seconds, it can't kill you at all, stacked recharge boosts mean all the powers involved recharge before their effects wear off, and the damage buffs stack to the point where a Defender's normally-weak attacks are best described as a rolling nuclear barrage.
The Legendary Weapon system in Ace Online boosts every weapon's performance significantly, but special mention must be made of the A-gear's bigsmashes. Compared to other high-performance weapons, the Legend Bigsmash gets far more overall enhancement than the other high-performance weapons: massive range and accuracy upgrades paired with its awesome damage make it outclass every other A-gear Legendary weapon, while weapons like the Arrows, Edrills, Bawoos, Blasts, and Sprints for the other airframes don't get that much of a significant performance boost when made Legendary. This is why Legend-friendly Bigsmash VVs sell for millions upon millions of SPI ingame.
Of special mention is the deadly build of Legend Bigsmash which is sometimes dubbed the Hax-Smash. With the right combination of Super Gamble modifications, the Hax-Smash has a massive boost to its rapidfire combined with respectable amounts of Range and Accuracy bonuses, which results in a gun with horrifying fast-firing and accurate high damage shots from incredible range.
In Phantasy Star Universe, Photon Art Axe Skill Anga Jabroga can hit up to 5 hit boxes twice in one attack at a high amount of damage and ignores attack accuracy. This however is balanced by its slow start up time (which leaves the player open to an attack), high PP use, and requires 99 Photon Fragments to obtain (only acquired by earning a grade of S on S-S2 missions), but to many players it seems more of a Fake Difficulty (Especially for the Fighmaster class, which increases overall attack speed with striking weapons).
In Gaia Online MMORPG zOMG!, the gimmick is that all your attacks come from rings. Everything. So originally, one could simply scrape up about 50,000 gold, buy eight Level 10 rings off the Marketplace, and sweep the game until you had the desired item (usually the coveted Sinister Scarf recipe the Final Boss dropped, along with the components to create the Scarf)... then sell off the rings to get your gold back. Upon learning precisely how broken this was, the developers put a stop to it by making rings "soulbound" and unsellable, nuking their original gimmick and forcing the players to work for their Sinister Scarves.
Another was that the bosses originally roamed around the screens, making it a simple matter to aggro one and then lure it over to where a couple of higher-level players would happily stomp it for you. OMGWTF, Kat's Doll, that giant Sand Golem, etc. They've since been placed in instances where nobody but you and your crewmates can get to them at a time, and your crewmates can't be above a certain level or they can't get in. Did I mention the bosses' difficulty scales, the higher your crew's average level is? Somehow the developers thought this was improving gameplay.
Now, entering an instanced area (including bosses) brings up a dialog box allowing the player (or the crew leader) to choose a difficulty level for the encounter, which affects enemy strength and thus the rewards for beating them. It does take into account the highest experience level among your crew, though.
The Turtle ring was designed as a "panic button" that boosts your defense to astronomical levels for a few seconds. However, the ring would increase your defense so much that enemy attacks would deal negative damage, resulting in the player getting healed. One of the devs admitted that this was due to sloppy coding that could have easily been fixed at any time, but for whatever reason they didn't for so long.
Guild Wars: Eye of the North introduced a set of skills available only to the PvE game (thankfully) that are so game-breaky that they make even the most difficult encounters from the previous chapters trivial. While Ursan Blessing has been given a lot of attention for this, the people who think it's the most overpowered skill simply haven't spent any time using Pain Inverter to make endgame bosses blow themselves up in less than ten seconds.
There are numerous team or solo builds in Guild Wars that break the game. Examples are 55 Monks, Perma-Shadow Assassins, the classic Domain of Anguish team, and the Paragon class.
Justified by Rule of Fun, however. Slogging your way through a two-hour dungeon filled with bullshit encounters is not fun. Pushing steadily and fairly quickly through it with 600/smite, however, is.
Unfortunately, Shadow Form got a massive nerf. While it still protects you from spells, it not longer protects you from physical attacks. Meaning you are now vulnerable to interrupts, conditions, and knockdowns from foes. Not only it ends massive tanking, but it also made Raptor farming and boss farming in Hard Mode impossible. Cue massive hate on ArenaNet.
Impossible with Assassins, maybe, Warrior/Necromancers can still raptor farm in < 1 min.
Other Notable game breakers are Bi P Necros and Imbagons. Bi Ps are players who reduce their maximum health to 1 allowing them to spam the skill Blood is Power on their entire team, normally the massive health sacrifice would make this impossible but at 1 health the 33% sacrifice is rounded down to 0. Imbagons meanwhile Spam the skill "Save Yourselves" to reduce damage to everyone but them by over 80%.
Players who use weapon-switching builds in Pv P. A typical warrior weapon-switching build includes multiple hammer knockdowns in quick succession mixed with sword interrupts, axe interrupts, ranger bow interrupts, and back to hammer knockdowns again. Breaks the game because there's very little anyone can do to stop the constant spam of knockdowns and interrupts. Against someone using this exploit, you're typically unable to cast spells, run away, move, attack, or do anything at all except die. Repeatedly. Another variation would be mixing assassin interrupts/knockdowns with warrior interrupts/knockdowns. The actual skills themselves usually do very little damage. However, since you're permanently paralyzed and unable to defend yourself, it basically amounts to Cherry Tapping an enemy to death.
Certain stances and other uninterruptable skills can alleviate this problem, but it's rather difficult to create a build that perfectly counters this and while still maintaining the ability to fulfill your combat role.
The Lord of the Rings Online has a Burglar's Trick:Dust in the Eyes, this debuff lowers a targets chance to hit and movement speed by 20%. While this is fairly balanced in PVE (where the effects don't stack and have attacks that ignore accuracy), in Pv MP no such failsafes are in place. This means that as little as two Burglars can make make a target miss 80% of the time, with 3 or more making even PC controlled Troll's completely and utterly defenseless.
Also, Burglars have Reveal Weakness, which increases the amount of damage the target enemy receives. Also, it stacks with itself, allowing multiple burglars to use it at the same time, making burglars a viable DPS choice.
Wardens are able to tank extremely well without a healer, healing themselves with their gambits. This way, they can solo or tank for group content without any healers.
Most second job change classes from Flyff have at least one skill that can be a game breaker if used correctly.
Psykeepers have the Satanology skill that has a high chance of locking an enemy in place for a limited time, and has a cooldown of 30 seconds. With a high enough INT, your binding time will exceed 30 seconds, allowing you to repeatedly lock an enemy in place. The spell may only have around an 80% chance of stunning at the maximum skill level, but you shouldn't need more than one or two stuns for any non-giant/boss monster.
It's also worth mentioning that Psykeepers have Crucio Spell, which deals damage to an attacker equal to double the amount they inflicted. People who like to abuse this tend to have extremely high STA so they have a ridiculously high HP pool, as they simply need to outlast their attacker.
Jesters can be broken one of two ways:
A standard Jester uses Yo-yos to fight. One of their skills, Hit of Penya, increases damage dealt by consuming Penya (the game's currency). Consuming enough Penya will cause it to deal massive amounts of damage in a single shot, often leading to one-hit kills in Pv P.
While they're meant to equip Yo-yos, Jesters can use non-Ultimate Bows because the prequel class of Acrobat uses both. Bows gain Hit Rate, Attack Speed, and Critical Hit rate from DEX, just the same as any other weapon, but also get their Attack from it. All classes except Jesters get 1% Critical Hit rate for every 10 DEX they have, but Jesters get 4%. With the right equipment, a Bow Jester who has just hit Level 60 and changed jobs can have 60% Critical Hit rate or higher, and will likely have equipment that further increases the damage bonus from Critical Hits. After enough levels, it's very likely they'll have 100% Criticals.
Billposters are the most common class seen in Flyff, both because they excel in self-sufficiency, and because they have the Asalraalaikum skill at Level 80. The skill consumes 100 FP and all of the user's MP to deal a single, massively powerful punch, with the power being relative to the amount of MP used. Many Pv P Billposters equip low-level armor with high MP and INT awakenings to increase their MP pool as much as possible, allowing them to 1-hit nearly any player in their way, save for high-level full STA characters.
While Blades aren't quite as overly powerful as some other classes, they have the innate ability to dual-wield one-handed swords and axes. This isn't as much of a problem until they can equip Legendary Golden Axes at Level 105 (or 95/100 if you use a Reduction item on them). Each of these axes increases Critical Hit rate by 20% and has a 40% Additional Damage of Critical Hits modifier on it. A Blade equipped with two of these and with a sufficient DEX stat will be a blur and tear an enemy to pieces.
Blades also have the Berserk skill which further adds to their Attack and Attack Speed. It's supposed to have the downside of being unable to use skills while it's active. Skills can't Critical though, so Blades tend to just use self-buff skills.
In the early days of Dark Age Of Camelot, the Midgard realm's Healer class had a line of crowd control spells that included a 'stun' spells which rendered a target or group of targets completely helpless for a short duration. At the time, the game rules allowed a player character to be stunned and stunned again while melee classes slaughtered them. This quickly lead to Midgard's complete dominance in RvR combat until a subsequent patch added a timer to how soon a stunned character could be stunned again, and lead to this era in DAOC history being known as 'Stungard'.
In Dofus, the Prespic Set is one of the few things in the game that provide a bonus to damage reflection. When the set was first introduced, reflection on equipment not only reflected damage back at the attacker but prevented damage to the wearer. This alone wasn't much of a gamebreaker, since the amount reflected was still fairly small. However, there was a medium-level monster called the Scurvion who attacked by stacking multiple weak DO Ts (of infinite duration) on your character from long range. Normally, a character would quickly succumb to the massive amounts of damage generated by the ever-increasing amount of DO Ts if he didn't kill the Scurvion in time. But with a Prespic Set, all of that damage went straight back to the Scurvion instead. You could start a fight with large groups of Scurvions, run off and make a sandwich, and come back to a victory screen with an undamaged character and oodles of experience. As icing on the cake, the Prespic Set also came with a Wisdom bonus, which meant EVEN MORE EXPERIENCE (1 wisdom = 1% more exp).
Within a week, a new patch came out where monsters took equipment and status effects into account, so they refrained from attacking if it wouldn't get them anywhere.
Runescape has protection prayers which can render many monsters to be unable to damage you. Most of the stronger monsters use multiple attack styles though, and you can only protect against one style at a time.
It also has the Overload potions, which boost your stats by a huge amount and remain fully boosted for 5 minutes, ignoring anything that would normally lower your stats. There's also a potion that heals a huge amount of HP while lowering your stats, combine these two potions and you can kill the God wars bosses as many as 10 times in a row without much trouble.
Fucking. Ice. Barrage. It's Aoe, Can be rapidly cast from offscreen, does 300 damage or more, and the absoloute worst part: It immobilises people for 20 seconds. Some pvp games can actually be successfully locked out by three Ancientspammers camping the respawn point.
In Dream Of Mirror Online the "Sara Nurse" is a pet that you can summon in addition to your regular pet, and spams healing and buffers to the owner. It's blamed to be the single reason of the reducing number of teams in favor of solo players, and borderline cheating. It doesn't help that Saras have been promoted for a long time, making them really common amongst high level players.
An Arcane Archer combining Arrow of Slaying (500 points of unblockable damage on a natural 20 that's confirmed as a critical hit) with Manyshot (Up to 4 shots per attack, each rolled differently) will result in 20 seconds of mass murder barring a merciful Random Number God.
Fighters and Barbarians combining True Sight (Negates all concealment, effectively granting a 100% hit rate against any non-AC build, which is most of them) with Improved Trip (Which has a DC so high only pure-Str or pure-Dex builds have any hope of saving) and Stunning Blow to stunlock people.
People abusing Sap (Paralyzes its victim for 18 seconds or until they take damage, has no save and a cooldown shorter than its duration) to keep people stuck in one spot until they get bored.
Clerics combining heightened Sound Burst (Stuns on a failed Fort save) and Cometfall (Massive damage on a failed Reflex save, Trips on a failed Str/Dex check) for more of the same.
This is to say nothing of the Bard's Fascinate (Requires a nigh-unattainable Will save to avoid being held if the bard has good Perform and lasts for nearly five minutes) and Irresistible Dance (No save, 24-60 seconds of helplessness) combo, combined with their typical inability to actually kill the people they lock up, which often makes the sound of a Fascinate being played enough to make everyone in a brawling pit immediately hunt down and kill the bard.
Spellcasters get the best of them, though - Not only do they get Irresistible Dance, but they also get Meteor Swarm (Deals Fire and Physical damage, the latter of which cannot be Evaded), Power Word Stun (Less than 450 current hitpoints and you can't move for a few seconds, no save), Flesh to Stone (Failed Fort save holds you in place for 30 seconds or so, no protection from it), and the oddity that is Chain Missile. Chain Missile is blocked by Shield and Nightshield just like all Magic Missile-based spells, but only if it's fired at the victim directly. This is handy as it deals Force damage, which there is no way for a player to reduce or resist once it gets through. However, Chain Missile deals splash damage that is *not* blocked, so a common tactic was to have a Shielded friend run up next to the intended target, then fire Chain Missiles at the friend. The friend's Shield would block the Chain Missile but the splash damage would go through the target's Shield and damage them normally. Firing it into a group of enemies had the same effect, though weaker since the splash damage spreads out. Of course, Wizards get a Reincarnation feat that lets them use a Magic Missile spell-like ability that bypasses Shield/Nightshield directly. Usually followed up with Power Word Stun.
Heavy Fortification, an item property that grants immunity to critical hits and sneak attacks, is something every character is expected to have by level 12. This makes sneak-based rogues completely useless in Pv P, as every competent player is immune to sneak attacks and hiding doesn't work in combat areas. It is not unknown for them to lose a straight melee with a Squishy Wizard because of this.
Truth in MMO'ing: Not so much L12 neccessarily, thanks to DMs each having their own style of treasure distribution, but, in the double digits any precision based class (Rogues being the most prominent example) in the tabletop version of D&D become useless as almost every major monster/PC/NPC is either naturally immune or easily makes themselves immune to their primary form of damage.
Entirely possible in Latale thanks to super puzzles. Here's how it works-you can use puzzles to add additional stats to your equipment, and cubes to reduce those stats. Puzzles reduce your equipment's durability, while cubes increase it, though by not nearly the same amount, if you're looking for some extra durability on a piece of equipment you just found. But super puzzles can have the potential to boost the stat by 50% more than a normal puzzle and have a lower cap durability that they can remove, just 10 compared to a normal puzzle's 15. So you can just keep using super puzzles...and then once you can't use any more, cube the item, which will only reduce the stat by the amount of a normal puzzle, and then enchant it again. Before super puzzles came out, it was rare to see an enchantment reach 40. After super puzzles, high end players focus on getting enchantments over 100. Of course the only real way to do this is to shell out real money for a mask and a TON of Ely for a Title to increase you enchanting level to 100%
Prime EXP Potions in the US version of the game increases the exp you gain from monsters by 500%. as long as you know what to grind on, at the least you'll gain about 5-20 levels in an HOUR. and this is possible all the way up to Lv. 193
YMMV, but combinations of Function Sets(fashion with stats) are this to an extent.
In Toontown Online, the Sound gag track has become a borderline example. If all four people in a battle have access to it, they have the ability to destroy a whole team of the most powerful enemies instantly because of the bonus damage that comes from more than one player using the same track, as well as the high accuracy of the track. The gags are extremely weak compared to other gag tracks' gags of the same level, so this strategy only works if all four have it. It can be done almost as well with only three, but there are lots of impatient players that only want to do anything if they can blast their way through it as fast as possible. This game-breaking tactic is actually more popular than playing with any kind of strategy, so people who don't like to play this way (or who can't play this way due to either not having the Sound track or not having the three most powerful gags in it) will actually have to let everyone else know and worry about being rejected because of it.
Granado Espada has the expert stance Shadow Sting, exclusive to Calyce. It hits twice per attack cycle (technically hits twice as fast than once-per-attack-cycle attacks), as hard as the omnipresent Flintlock (the stance of choice for DPS), as far as it as well. Then the skills, Eagle Eye can be used with Cat's Eye, to make your Penetration stat shoot up to 100+, which kills 100+ of your Target's DEF, and practically makes both Block and Evasion useless (1 Penetration kills 1 Block and 1 Evasion, and since both Block and Evasion has a ceiling and Penetration doesn't). Heart Breaker inflicts Drain, which drains your HP very fast. Back Step Inferno ignores 40 more DEF, hits multiple targets, and inflicts Burn (making him unable to attack). Bolt Sequence is Back Step Inferno, minus the Burn but hurts a lot more. And then there's Shadow Dawn, the tank killer, has a 175% multiplier on anyone that's wearing Metal armor, and when coupled with the veteran stance Sagitta skill Silent Move, it's damage will be boosted up to double. But then again, a bare Shadow Dawn will be enough to kill any target that has below 50k HP. And the worse part about this stance? All skills are fast cast, and can be used in tandem with Silent Move, which means you'll never know when and where Calyce will hit you.
In AdventureQuest Worlds, there used to be a bug regarding the mage class. It had an ability called infusion, which regened some mana for each hit on the target monster. But the mage also had an ability called explosion, where the damage doubled for each cast of infusion on the monster. Therefore, you would see groups of ~10 low-leveled mages +1 high-level mage INSTANTLY destroying a powerful boss mob, as seen here. Sadly, this got nerfed.
Star Trek Online, at the end of the "2800" Featured Episode arc, issued a reward for the final mission in the form of the final piece to a set, a Jem'Hadar shield with a passive brace for impact effect. The passive brace for impact inherited the percentage chance for a shield regeneration proc offered by slotting a shield distribution duty officer (or several), resulting in players attaining near-invulnerability and potentially lagging everyone in the instance to an unplayable standstill from taking too much fire. This has since been patched out.
Several of the Mr. Store Item(s)-Of-The-Month, special items that are usually bought with Mr. Accessories, which requires donating $10 or more to get, are made of this, as they make the game exceedingly easy to speedrun, and acquire better ascension rewards as a result. In addition, while Mr. Accessories cannot be pulled from storage in a Hardcore run, they can be bought during said run, and provide +15 to all attributes for each one equipped. Some of the most powerful items include:
Boris' Helm, a helmet that can be used in the "Avatar of Boris" challenge path, the first Mr. Store equipment to be usable in a Hardcore run, and gives massive stat gain boosts, monster level increases and weapon damage.
The Loathing Legion knife, a multitool that gives 20 different changeable benefits, including stat gains, item/meat drop boosts, doubled weapon damage, more rollover adventures and more combat initiative.
The Clan VIP invitation, which gives a player access to any clan's VIP Lounge, that, depending on the furniture contained within, gives several helpful buffs each day, a free present every week, five free HP restores each day without consuming an adventure, a pool/shower that each give special items/equipment, a crane game that holds a unique familiar and enchanted equipment, and a fax machine that lets the player face almost any enemy that's ever been designed for the game, regardless of whether it can't be accessed normally anymore.
Hodgman's imaginary familiar hamster, which can only be acquired if the Hobopolis clan dungeon is completed in 1100 turns or less. The process required to get this is incomprehensible without a guide, and there isn't even a good chance you'll get it on your first or fifth try, but if you do, it can be combined with other pieces of Hodgman the Hoboverlord's gear to get the most item/meat drop increasers and HP/MP regeneration of any set in-game, allowing you to make massive amounts of cash in a short time, and trivialize the rest of the economy. The only downside is that the set requires you to have a minimum of 200 in all three of your stats before you can equip the outfit.
The main cause of the downfall of the original Planetside was the introduction of the infamous BattleFrame Robotics in the Aftershock update. These things worked like big, stompy robots with the durability and the weapons of a heavy vehicle, but only requiring a single person manning it to get the most of it, whereas other vehicles required one person driving and other people manning the big guns. As a result, a single soldier piloting one of these could mow down lots of enemy soldiers and vehicles. This encouraged solo play and killed the entire point of the game. Players gradually abandoned ship after the introduction of these things.
SD Gundam Capsule Fighter has the Enhanced Close-Range Protection, which increases your defenses from melee attacks and gain the ability to no-flinch attacks, meaning a unit could race in, attack you all they wanted, and you could turn around and smack them instead. Most PVP rooms, especially 1 on 1 melee rooms, outright ban units that use this.
The Super/Hyper Modes used by certain Mobile Fighter G Gundam units fit into this as well as they, too, had no-flinch properties.
Deathscythe Hell (EW) was a majorly notorious Game Breaker unit, especially prior to the Generation Six update. Its melee power was incredible and since one of its attacks rendered it invisible, all a DSHEW player had to do was turn invisible, race up to a unit from behind and nail one melee combo. Boom - the unit was dead.
Thanks to Generation Six, a Level 12 Mobile Suit Gundam SEED unit with N-Jammer Canceller becomes this. At Level 12, the unit gains a part that allows players to double jump without using extra boost. N-Jammer Canceller allows players to dash infinitely without running the risk of draining their boost meter. There's also a Good Bad Bug which allows players to continuously jump into the air after melee attacking as it resets the jump counter. Thus, units like the Freedom Gundam and the Justice Gundam can essentially "kite" across the battlefield. The Infinite Justice Gundam makes this worse - he can dash infinitely across the sky, throwing beam boomerangs at his opponents without the threat of being hit once. This has lead to a lot of Rage Quit.