The Assassine's are sometimes Invisible, if they jump on someone.
Also sometimes u get stunned by an assassine (the purple ring), but there is no assassine and your stunned till you change hero.
Also u can't unequipment your gear with square.
here is a short vid of these nasty fellas in action
We believe we've fixed this issue in one of our previous patches. If you're still seeing this, please submit a new bug report!