When you rapidly casting it on 2 and more units on conditions provided below, it happens to misfire. I've observed since the launch and I thought I've reported it, but can't find an actual thread.
1) Melee\ passive charge.
2) Multiple Infinity Portals.
3) Upgrade providing 2 sould gems.
I've confirmed this in skirmish as well, when wiping out anything but enemy's HQ and Core. I didn't get to observe this bug on a single IP, even with 2 Soul gems researched (I've got steak of 2 positive recalls and dropped it), as well as on Wraithgurds exercising ranged combat.
May you look into it? I guess, it was the main reason why people have choosen not to build Wraithblades past times.