When placing a cable (tried with copper, HV and gold) at y=256 (It does not allow me to place a normal cobble block at this location) it causes the client to crash, the server remains operational. Any client on the server will crash when this block is then downloaded from the server. (if you are far away it remains fine). I am unsure if it other IC craft items crash it.
To fix the problem I had to use MCedit to delete the block.
My server runs:
The client runs the same.