Ok here i am trying to contribute what i can.
Searched and couldn't find anything similar so here goes.
When dueling in dalaran sewers near the arena masters there are pipes coming out with piles of rubbish under them. At the middle pipe to the gear vendors there is a 'ridge' in the wall and a dip as the piles of rubbish go into the corner.
The general location of the area.
Stand here when dueling and fear/death coil etc etc.
Well here's the trick, duel your opponent and lure him/her into this corner when they are there use your spell which causes loss of control of said opponent's character, be it polymorph, blind, scatter shot, death coil or fear.
Your oponent will fall through the ground and land in the un-instanced Dalaran Arena below and if they have a hearthstone cooldown, they're stuck.
This method could also be used to test positioning in the arena and see how mis-shapen the boxes are and how far a healer would have to come out to be able to heal without line of sight issues, but i prefer just using it as a practical joke.
Enjoy.