Steps to reproduce:

1. Dedicated Server only (single player instance will not display the bug)
2. Find a guard and attempt to hire them.
3. Upon accepting the hire, the server will crash and the client is unaffected.

Crash Log & Findings

The server appears to call a client-side only method in the packet handling. The crash log is attached.

crash-2016-04-26_23.22.26-server.txt

I traced the call to InventoryPlayer::getSlotFor() which is tagged client side only.

Forge & Minecraft versions

Forge #1865
Minecraft 1.9

Investigation
This only occurs when playing on a server with MCA. func_184429_b is getSlotFor(...) and this method is @SideOnly(Side.CLIENT) which causes the unhandled Exception.