[takato14]

It's inside the arm. He conveniently forgets all complicated things are made up of many humble parts. To date I've never heard of anyone having a problem with these pins, or anything related the construction of the HD800. Paint? Sure. Tuning? OK. Failing drivers, broken parts, things falling apart? Not yet.

I don't think that's a cotter pin, it's something else
 
Also weren't there a few driver failures?

 

[RUMAY408]

Shear pin is a better description.  Popped one out of my refurbished HD800.  I was so impressed with the sound I spent more bucks for a new pair from Crutchfield after they refunded my money for the first one.
I literally laid them down and they fell apart I put the pin back together on the hinge which is held in place with a spring.
A good warranty is a must for high end HP's.