I tried searching for this and didn't come up with much. Just may be common knowledge but for those that don't know, grats.
If you don't have the key for an instance, stand in front of the locked gate and blink. You'll be inside the gate, turn around and run back towards door and you'll zone into the instance.
I was locked out of TK-Arcatraz the other day and blinked through. I don't have the key on that character, and no other characters around.
Hope this helps someone.