Ceremonial Void Sword being sold for 300 ACs instead of 1500, and then them bending over backwards to appease the people who got away with only paying 300 for no reason
Arcane Dark Caster class butthurt that forced them to change it from the original DC's skills
Also when tricky exploiters bought Shadowed Dark Gunner, got ADC for free, and then sold SDG basically getting the class for 200 ACs -- nobody punished for that
Dark Caster class and Dark Caster X
Blademaster Assassin and Legion Blademaster Assassin
100% drop rates on test server... 3 or 4 times, not to mention 24x boosts and the gold cap thing
Wheel of Doom rereleasing Quibble stuff, and then subsequently Wheel of Doom leaving
SUATMR making people mad about new players winning Alpha Pirate
The whole thing involving Nulgath gen 1 and the whiny babies that continue to plague the landscape over "rare" items with no actual scarcity or value...
(side note, they actually can track AC items being used in quests now, but it'll never happen again thanks to the merge cheat)
Oh wait, LQS shops are a thing. People who managed to shit that up by buying and reselling items to artificially decrease the amount available, eventually leading to the new site.
Shadowblast Arena used to ask for 100 Seals per Emblem, and some other stuff that made it about equal to getting Armor of Awe level of tedious before they slashed the requirements
There was also that time with the Synderes Blade of Doom where everybody found out that
the guy was a botter and that was mildly controversial for a little bit
Hardcore Paragon Pet needing
over 64000 hours to complete
DvN war Legion quests not needing Undead Warrior
Probably some other ones but this is a decent list