Hello there OwnedCore,
I've done my homework, and looked up if there was a thread on this forum about it, and the only thing I could find was this thread on the official forum:
Deathgrip through the floor in temple of kotmogu - Forums - World of Warcraft
I could achieved this in Dalaran Sewers and Kotmogu so far.
In Dalaran, I was able to death grip a warrior (blue cross) from the red cross. You really need to find the perfect angle for it.
Dalaran Sewers
In Kotmogu, I was able to death grip a mage (blue cross) by JUMPING from the red cross.
Temple of Kotmogu
This is probably nothing new for most of you, but some DK cousin out ther might be interested in practicing this glitch (because I don't think it is intented). Enjoy!