That worked, didn't it.
I think we need to require anyone with something not in the normal army books to have a copy of the relevant rules for their opponents, be it the book itself or even better a copy of the page(s) the opponents can have for referrence. This is an open list event and lists are exchanged before every game.
In particular I'm thinking Monstrous Arcanum, Tamarkhan and Storm of Magic. Not everyone has these books and some of the models have unusual rules.