World of Warcraft Classic has been having problems with layer hopping, a technique that would allow players to farm experience and materials easier. Blizzard found a solution by increasing the cooldown time before being able to hop again.
World of Warcraft has been a blast of nostalgia due to all the cool things from back in the day getting a resurgence but some of the newer systems are still embedded. One such system is layering. For those unacquainted with the term, it's basically Blizzard allocating players from one overpopulated instance into many copies of it.
For example, if one server has 300 players in the Barrens, it will impede the possibility to complete quests for all of them since everyone will be fighting over the limited number of creatures and materials that can spawn at one time. Layering then kicks in and distributes those 300 players to six layers of 50 players each so it's now all crowded. Keep in mind the numbers are not exact ones picked up from Blizzard, but rather random ones for the sake of having an example.
Some players, however, found a way to exploit this system in order to farm experience and items faster. If a player kills a group of enemies or mines a node in one layer, they can group up with a player from another layer which will automatically transfer the first player to the other one's instance of the map.
Therefore, all the mobs that were dead a moment ago will be alive again and the mining node ready for giving up more metals. Players started layer hopping and the technique was most prominently featured when Jokerd, the first player to reach level 60 in Classic, started using layer hopping to cut down on time (roughly an hour and a half) in the last part of his power levelling.
Players soon started reporting the economy being ruined, LFG chat being spammed and banks filled with valuable metals like Thorium. Blizzard first found out that many of these reports were false but took action against layer hopping regardless.
The developers have introduced a layer change cooldown through a hotfix. Now, when a player changes a layer, their cooldown before doing so again will be a few minutes but it will grow if the same player keeps changing layers. On the flip side, players who don't employ this technique will not have the cooldown to begin with and if they do get some, it will wear off the longer they play in their designated layer.