Some mobs have delayed attacks (the gorilla's slap for example), if you kill it midway through its attack, you will take damage anyway.

This also appears to apply to emus.

I've updated the title as this is still relevant in newer versions and affects more than just death - basically, the target needs to be tested to ensure it's still valid after the delay if a mob has a delayed attack.

It also appears as if the gorillas specifically ignore range checks with their delayed attack, you can take damage while backing away from them.

+1 on this, on the latest versions of forge, mod, and citadel

I'd just like to add that this also applies to the mosquitoes; their drinking attack can hurt you after you've killed the mob.