(maybe they wont remember to fix this for live but who knows)
A warlock can spec the 41 point Demon talent for Felguard, summon one then respec with a felguard already out. They can have any spec with this felguard and go anywhere that would not cause the pet to despawn. Flying, or arena would cause the loss of the pet (sadly ) but it gives any spec a extreme advantage of using this powerful demon with any other specs of their choice and could cause a lot of trouble.
Im not sure if the bug where the warlock gets mind controled by a friend and gets to keep the pet no matter what is still around on PTR (always crashing so I cant test) but if it is than that could make an awesome combination with this.
Found this info from some bug thread on PTR forum but I added the last paragraph to it.