The hammer has finally come down.
No more 10-man dungeon farming! This was hotfixed by Blizzard today on September 1st.
This method involved entering 5-man instances such as Scarlet Monastery and Zul’Farrak and making a raid group of 10 people. A bug (or not remembering to flip a switch) gave raids up to 10 people bonus dungeon experience upon killing a mob, instead of incurring a raid penalty that reduced XP.
You can see it in action here, where the hotfix was deployed mid-trash pack kill.
Some are calling for individual player rollbacks due to bug exploiting, but overall most are too involved in their own Classic WoW experience to be that bothered.