The problem:
While doing heroic modes in raids, you will be trying the encounter for like 40-50 times before you beat it, eventually killing it in normal mode to continue going on and finish the farming.
What is happening now is that when you try a boss in heroic mode and kill him in normal mode you some times get saved to a heroic ID, leading to only 25 players being able to join this raid, as this prevent anyone not locked to this ID to join, even if they didn't have a normal raid ID.
The Solution:
It's nothing hard to do, let's say you spent the whole day trying magmaw heroic with 30 of your 40 raiders and them decided to kill him in normal mode, so 10 players never joined the raid and 5 of the 30 players who did, are not inside the raid at the moment, 25 players will get a HEROIC raid lockout. What needs to be done if you choose to kill the boss in normal mode is have the raid disbanded and reformed by a different person that's was not the original raid leader(you can pass him the lead after you reform your group). Also, don't change from normal mode to heroic(heroic > normal is OK) inside the instance as some times this bugs and some players get teleported out, being lock to non-existent Raid IDs.
Basically when you're going to kill the boss in normal mode, have the raid disbanded and reformed by a different person to prevent you from being locked to non-existent raid lockouts.
Hope this help people who are planning to do heroic modes or to avoid people having their lockouts ****ed, like we used to have on past 2 weeks. Now that my guild is using this method, we haven't had a single problem in our whole week of raiding.