Not sure if this is working as intended.
I just respecced ar/prep and got riposte. It's pretty sexy. anyway, I was in AB 5capping some noobs when I found this. I parried a warrior's attack, then he died, and I was able to riposte a different target. I've tested it a bunch of times, and even if this isn't an exploit, I had no idea.
If you're able to use riposte, you can switch targets and use it on someone else. It's especially useful when you parry a pally/something and turn to the warrior who's ****ing up the healer and riposte him.