I've reported this before on different sets. This time it's a Street Justice/Ice Armor Stalker.
I can't duplicate the bug, because I haven't been able to determine any sort of pattern.
Once in a while Assassin's Strike will do no damage, but otherwise appear to have hit. The mob is notified, am I no longer in Hide, the Hit animation plays, but there is no damage and the combat log shows NEITHER a HIT nor a MISS. Just "You activated the Assasin's Strike power." and then me taking damage because the mobs now know I'm there.
I've seen this same bug on other Stalkers with different primaries and secondaries.