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