We've recently noticed this. It seems to happen most frequently with the special strike type attacks (I. E. The rogue 32k hit). You can frequently see the damage on the other character's chat window as well.
It doesn't cause any detrimental impact on game play (it's not clear if it causes agro for the wrong character). It's quite funny seeing a mage or cleric backstab for 32k though
Also happens with the beastlord feral swipe (8.5k). I believe these are unique to THF. It seems to happen only when the attack is a kill (as noted above).