I don't believe this one has been posted yet.
This place is located Second floor of the horde base in Twin Peaks..
All you need to do is put your face in the corner of the wall and jump
You'll know when your in, then just turn yourself so the flag is hiding in the wall
Had like 4 allys pass me in one game!
Ps. I found this bug first, i got the first flag capp of the game and found it then told that goblin to do it too :P