Due to the (limited?) system of the raid system, especially in the better dungeons, raid groups are granted a raid instance ID that gets reset every Tuesday during maintenance. The idea behind the instance ID system is to bar particularly greedy raid groups from hogging the raid dungeon for themselves.

But the system also has its loopholes. Take for instance the way the IDs are saved. Group leaders are the principal basis for identifying the group instance. Their IDs are the groups' collective ID, so the ID reflects constantly throughout all the members of the group.

Even when the group splits after the raid, each member keeps that ID until they join a new raid group with a new leader and adopt his unique raid instance ID. In turn, the new ID gets saved to their character and keeps that ID until changed by another instance or reset by the maintenance.

But if each member becomes a leader of a group, then the instance ID cascades to an even larger number of people, and increases the chances that the raid instance they should be in will get jacked. Instance jacking, the process behind it and the horrors that can develop from it, is best known in World of Warcraft as "instance griefing".

Can GMs do anything about it? They certainly could. But will they do anything? The answer is no. It's apparent that the act of "instance griefing" is unethical, but there is nothing about it being illegal - at least in that sense. It doesn't qualify as "griefing," but more or less a rampant annoyance. Because the system is limited in a sense that it was developed that way (and also has the occasional bug), it couldn't be considered a cheat nor an exploit. People will not act around the system, but go within it in order to wipe your raid instance.

The system was supposed to stop people from keeping the raid dungeons to themselves, but it seems that they have found a way to achieve the same effect, even with a system in place. So now people can go about jacking instances from people who worked so hard to prepare for and the raid is becoming more of a chore and a headache than a fun run down the gauntlet.