First up, a disclaimer - I couldn't find a single other post or article about the mechanics I'm about to talk about. As such, everything here is written from memory and about a game I played 10+ years ago, so expect some inaccuracies. If you have any trustworthy resources on this topic, let me know on Twitter (@tokphobia, link in the side bar). Now that's out of the way, let's start by talking about MU Online and one of its more interesting systems.
Gaining XP in MU Online
MU is pretty old, launched all the way back in 2003, with gameplay that boils down to an action RPG taking place across multiple server shards, similar to Diablo. While the game is relatively generic compared to today's titles, the thing that stands out is the shared party XP mechanic and its implications.
When in a party, all XP would be multiplied by the number of people in that group, however it would be distributed proportional to each member's level. Assume you have a party made up of two level 3 players and one level 6 player. This party then kills a monster worth 100 XP. Since there are three players in the party, the game will have a total of 300 XP to distribute. Distribution is proportional to each character's level, so the level 6 player would get 150 XP, while the other two players would get 75 XP each. Unfortunately I couldn't find the exact formulas on-line, so the developers actually calculated things a bit differently, but the base idea remains - XP was multiplied according to the number of people in the party, then distributed proportional to each players' level.
The next important fact was that the game's content was split into multiple regions, with monsters of various levels clustered in certain areas of each region. You were expected to go through each area of each region, in the order set by the monster's levels, grinding for XP and items in order to survive in the next area. Pretty standard stuff.
Finally, the game really permitted a sort of AFK farming. One of the game's classes, the Dark Wizard, had an AoE spell that could cover the entire screen. Coupled with the fact that the game would automatically chug a mana potion if you ran out (as long as you had some in your inventory), you could grind monsters all day by just holding down the right mouse button. In fact, that's what most people did - auto-clickers that would spam the right mouse button or, for those looking for a slightly lower-tech solution, pinning down the button with a toothpick.
Players have a nasty habit of not doing what the designers intended them to do. As was the case of the Destiny loot cave, the easiest option of getting loot and XP is often the one players go for, even if it's not necessarily the most fun to play. In MU's case, that took the form of organizing "level boost" parties where the name of the game was finding as many newbies as you possibly could. In MU's second region, Davias, low level players would run into very high level characters, offering parties that would level them up very quickly. All for free. Obviously most people accepted the offer, as did I.
Those high level players were essentially using the game's party XP distribution mechanic to their advantage. By taking another four low-level players in their party, they would essentially be multiplying the XP granted by monsters by 500%. In addition, because their levels were much greater than the other party members, they would essentially get almost all of the XP. That way they could get much better XP than they could by killing monsters their own level, all without even paying attention to the game since they could just AFK farm, only checking in periodically in order to get some fresh low-level players in their party.
It wasn't a scam though, as the party would kill the toughest monsters in the region at a very fast pace, thus really granting the low level players much more XP that they could get on their own. Everyone benefited!
So what's wrong with that?
Well, the whole thing only works if there's one really high level player and four really low level players, otherwise it's not worth it for the guy doing all the killing. That means that low level players would just be discarded from the party once they levelled up too much, in order to be replaced with someone that's lower. That meant that the discarded player would have reached a certain level too quickly, without actually grinding appropriate items, and would essentially become "stuck". He couldn't kill monsters at his own level, although he could kill monsters at his previous levels but those granted too little XP, so most players wound up going around begging for parties or at least trying to steal items off the ground while dodging monsters. Sure, they could have just killed monsters way below their level and slowly grind the loot they need, but that meant that they had wasted all that time power-levelling.
It was a strange feeling being in a party, seeing these players that are higher level than you but having the exact same gear, moving from group to group begging for a boost or trying to pick up any items on the ground. Eventually players would either quit the game, spend enough time stealing items off the ground or slowly grind their gear level the old fashion way. Some would even return much later on to boost their own group of newbies. Either way, things were not working as intended.
Even the smallest mechanics can have a big impact on your players and how they interact, so make sure the easiest way of farming content is also the most fun and rewarding for everyone.