I have the rulebook, FAQ, and combined codex on my tablet that I can bring. I agree 100% that mistakes happen, and with the format of only one game per match, due to the length of games, I don't want to ruin someone's fun by being too harsh. I think the important thing is to try and have everyone enjoy the event.
So as a general rule, warning first, loss second, and it is okay to penalize them in other ways, such as mana, to try and balance the mistake.