Arceus! 

Its because your using the Extra pistons mod, I can tell because the piston is pink.

When you get a gold helmet it is glitched as a different piston, your client crashes because the server does not support that mod, its like any other mod that has new blocks.

How to fix this:

• Completely delete your .minecraft in %appdata%
• Keep 2 copies of .minecraft one with the mod so you can use it on single player, not multiplayer.

I know its a bit of a hassle, but you will have to change your .minecraft every time for the mod.

I will try contact the creator of the mod and inform him of this issue.

Hope this helps!

-Waterworth

Nah, it won't be too much of a hassle, i barely pay singleplayer mode anymore cuz your server is too awesome

Theres some glitch on my computer that makes golden helms look like piston heads from the more pistons mod, and the server crashes every time i pick up a golden helm, i don't know if this applies for diamond too...i never tried...well please, if you know how to fix this, tell me, i hate force updating a ton of times and putting my original .minecraft pack in %appdata%.

Heres the piston head, thanks TidalMewMew for helping me take this picture: http://tinypic.com/view.php?pic=20raetk&s=6 

I dont know how to post pictures instead of links to pictures XD new to this...