There is a quest in FNV where you have to save some captives from the Legion. If the captives witness you killing the legionnaires, they will actually report you to the Caesar faction, dropping the player's reputation right down to HATED with that faction. I don't know what Obsidian was thinking releasing a quest without testing it, they made the captives IgnoreCrime, but I don't think they actually understand how IgnoreCrime is working. Talking about lack of communication between the engine coders and scripters departments, huh? IgnoreCrime will NOT prevent NPCs from reporting crimes committed by the player or other NPCs. There are only two ways to prevent crime reports: If the NPC who detects a player commit a crime is hostile to the faction against which the crime was committed and if the NPC is a player teammate (in which case detection will not trigger at all, so the NPC doesn't technically witness the crime). Now talking about fixing this particular quest. How to prevent the captives from reporting the murder crime to the Caesar faction? Well, we could set them hostile to the caesar faction, which is probably not such a good idea, since the guards guarding the captives will start shooting them on spot. A second alternative is to set them as temporary player teammates, which is of course also only possible if the player is not hostile with the caesar faction, otherwise the guards will attack them as well. The only other thing I could come up with it damageav perception 10 to reduce their perception radius somewhat, but it's actually impossible to disable detection completely, even SetUnconscious will NOT prevent an NPC from running their detection routines. You could also put some textures around the captives to prevent LOS between them and the player (for example by putting them inside a cage or something). This way perception wouldn't trigger either.

So what happens if you set the captives as temporary teammates and then sneak up on legionnaires and sneak-shoot them? Because there's nobody there to actually report the crime, the player does not lose any reputation with the caesar faction. You just need to kill them all before they can detect the player, which means one shot per NPC to the heads with a silenced weapon. The problem here is that the player playing the game thinks in human or let's say ethical terms, while the game engine thinks in technical terms. I hope you see the discrepancy here, Obsidian, and the problems it causes: It doesn't make any sense to a human being that a captive reports his rescuer to the faction that captured him, but it makes sense to the computer engine.