A bit more clarification of what I meant about the damage systems above. Let's say you have 3 wounds left, and then get hit in the arm for 5 wounds after armor and such.
In WFRP, you would be at 0 Wounds, and roll a d100 on the critical table with a Critical Value of 2 (for going 2 over your Wounds). This would give a Critical Effect of 2 to 7, which you would then look up on the Arm critical table. This would lead to a variety of results, ranging from just being unable to use your arm for a round, to turning your hand into a bloody mess that might never be usable again and having a 20% chance of dying from blood loss each round.
In Dark Heresy, you'd just look up the "-2" result on the arm critical damage table, which probably isn't all that bad (I don't have the table here to check).
Now, let's say you get a second hit for 3 points of post-armor damage. In WFRP, you never go below 0 Wounds, so you'd treat that as a new injury, rolling with a critical value of 3 (for 3 points left after your Wounds are gone). In Dark Heresy, however, you'd keep going lower, so you would check the critical tables for a value of -5.