In the current combat snapshot (8c), it seems that all attacks grant one more iframe than intended, or weapons reactive one frame sooner than intended.

In previous versions, it is possible to hit a mob once while jumping and then again while falling. This second attack is triggered while falling, and therefore is a crit, dealing more damage. This 'late crit' is only possible because the mob has iframes while being attacked. All of this is intended behavior.

While difficult to replicate in the snapshot, it is possible to hit a mob with a normal attack, and then hit it with a critical attack at the very moment the player can attack again. If the attack is not frame perfect, then the target will take critical damage as normal. If it is frame perfect, this creates the 'late crit' that was possible in 1.8.

Because I was able to kill an iron golem with this, it is likely not just a visual bug.

Because of the nature of weapon cooldowns, these high-skill attacks actually end up hurting the player who pulls them off, so it is probably worth fixing (unless the root of the problem is more involved than an off-by-one error). I have tried to hold down the attack button after landing a 'late crit' to see if this is accurate, but I haven't been able to pull this off.

If you wish to turn this into a feature, it probably should not consume the weapon's cooldown.