So while playing the game and visiting forums I frequently see people misinterpret various rules (at least to my knowledge). the most common of these (online at least) is the rule for Feel No Pain. The argument I usually see goes something like this:
"If the AP of my weapon ignores your armor save you don't get FNP."
First let me provide an example of why this is completely ridiculous. Ladies and gentlemen I present, the lowly Plague bearer
So lets look further at the FNP rule.
Feel No Pain
"If a model with this ability suffers and unsaved wound, roll a dice. On a 1 ,2, or 3 the model takes a wound, on a 4, 5, or 6 the injury is ignored. This ability cannot be used against wounds from weapons that inflict instant death. Neither can it be used against wounds from Ap 1 or Ap 2 weapons, power weapons, and any other wound against which no armor save can ever be taken. (Power fists, Dread close combat weapons, rending, perils of the warp, dangerous terrain.)"
Breaking down the rule we get
If a model with this ability suffers and unsaved wound
The term unsaved wound is important, this could be part of the contention, but it is usually not, brought up. In general any wound that is not saved (including those against which you don't get a save) would be considered unsaved. This appears to be how the INAT FAQ interprets this rule Though there is no specific reference to this in the rules. In fact the only reference at all is in the allocating wounds box at the bottom of the page 25, where it state that the unit has suffered 2 unsaved wounds and one wound with no armor save from a melta gun. I can see an argument being made off of this that unless you take a save of some kind (including invulnerable) you don't get feel no pain. However, I believe that the assault results section of the book (p.39) supports the idea that any wound that ignores armor saves counts as unsaved (otherwise power weapon wounds don't count toward combat resolution as it only mentions unsaved wounds)
This ability cannot be used against wounds from weapons that inflict instant death.
This section of the rule is fairly clear cut and subject of little argument. If something causes instant death (either double toughness or special rule), then no Feel No Pain.
Neither can it be used against wounds from Ap 1 or Ap 2 weapons, power weapons, and any other wound against which no armor save can ever be taken.
Now we get to the most debated part of the rule, especiall the part in bold. Everyone understands the first part of this statement, no Feel No Pain against AP 1 or 2 or power weapons, pretty straight forward. The part that many people get wrong is the bolded statement, "no armor save can ever be taken." The misreading of this rule is that it means if you don't get to roll an armor save you don't get feel no pain. In other words if the AP of my weapon defeats your armor save, it also bypasses feel no pain. This is incorrect reading: no armor save is not the same as your models armor save. No armor save means that there does not exist an armor save which may be taken against said wound, or that the wound bypasses all armor saves in the game. Your models armor save is the specific save of the model in question. A more clear wording of this statement would be a wound against which a 2+ armor save cannot be taken against.
Again consider the plague bearer, if no armor save = your models armor save, then plague bearers (who have no armor save) can never use their feel no pain special rule.