The rules for re-rolls says that “re-rolls happen before modifiers (if any) are applied). This has some curious side effects.
The one I’ve met the most is that if you have a -1 to hit something and you can re-roll rolls of 1 to hit then if you overcharge a Plasma weapon it will explode on a 2 and there’s nothing that you can do about it.
But thinking some more this also happens to wound rolls when you have positive modifiers. For example consider my melee units. I’ll often try to manifest Blades of Putrefaction on them giving a +1 to wound and then I can also use the Veterans of the Long War stratagem to give them another +1 for +2 in total. Now complicate it further, as I often will, by having my Warlord nearby with Arch-Contaminator as his Warlord Trait. This allows me to re-roll all failed wounds for Plague Weapons. But I have to do that before I add the +2.
So let’s expand the scenario a bit. The melee unit mentioned above is a squad of Plague Marines armed with a Plague Knives and they’re attacking a squad of regular Space Marines. So that’ll be a S4 attack against a T4 Marine, I’ll wound on a 4+. Normally I’d just say “OK, I’m wounding on 4+ with a +2 so I’m looking for 2+ on the dice rolls.” But that’s wrong.
What I have to do is to roll my wound dice putting any that are 4+ to one side. Then I grab everything that is under 4, even the 2s and 3s that would eventually have become a wound, and re-roll them (that’s presuming that I’m fast rolling everything).
This will cause my chances to wound to go down, I’m re-rolling dice that would otherwise have caused a wound and there’s a chance that they will now fail to wound.
On the positive side when I’m in this sort of scenario I’m also looking for wound rolls of 7+ to trigger the mortal wounds that Blades of Putrefaction gives me. So I now get another chance to get the 5s or 6s that would trigger that when I’ve got that +2.
However the simpler scenario of just having Veterans of the Long War will happen more often. And I’m going to have to remember to do this right in the future.