I've found a couple situations where a couple rules kind of collide, and I'm not entirely sure which rule takes effect first. I hadn't seen anyone asking about them yet, so... here are two situations where I'm unsure which rule would be considered first.

Force Weapons v. Feel No Pain:
Force Weapons - p36: "If a Psyker inflicts one or more unsaved Wounds with a Force weapon, he can immediately choose to activate it... If the test is passed, all unsaved Wounds inflicted by the Force weapon that turn have the Instant Death special rule."
Feel No Pain - p35: "When a model with this special rule suffers an unsaved Wound, it can make a special Feel No Pain roll to avoid being wounded (this is not a saving throw). ... Note that Feel No Pain rolls cannot be made against unsaved Wounds that inflict Instant Death."

The exact wording for force weapons indicates that they can be turned on immediately after a save is failed to cause Instant Death. Feel No Pain states that it cannot be taken against wounds with the Instant Death rule. The question is, do you check for Feel No Pain before force weapons would be turned on (as part of rolling saves), or do you immediately turn on force weapons after failing an armor save, thus never getting to roll Feel No Pain? My take is that Force Weapons would be turned on as soon as the saves are failed, but before they're "suffered" by the model they're allocated to. Particularly, this would be visible in a case where all models have the same saving throw, but only some have FNP - the saves would be failed before they're even allocated to the models to determine if they have FNP. That said, I've got an obvious reason to want Force Weapons to win out there, so I'd rather clear it up beforehand than have a debate when I try to zap single-wound models...

Instant Death v. Eternal Warrior:
Eternal Warrior - p35: "A model with this special rule is immune to the effects of Instant Death."
Again, I'm curious which rule wins out there - can FNP be taken against ID wounds if you are an Eternal Warrior, since you ignore the ID rule, or since the wounds have the ID characteristic, is FNP skipped even though ID would not actually be inflicted? I really have no idea which rule takes precedence in this case, and can see equally valid arguments for either side.