This may have been mentioned already, but I have seen a bug with the hidden power that has occurred multiple times. The player or companion who is hidden remains hidden after attacking. I saw this happen in pvp when facing a swashbuckler, he remained hidden after attacking with assassin's strike. Also, just now, El Toro remained hidden in a different fight after attacking me. In both cases, the hidden person remain untargetable after landing an attack
It doesnt happen every time so I am not sure that I can reproduce it, but again, what happened was:
1) The opponent pirate in a PVP match used a hidden power and attacked one of my companions. After the attack, he was still hidden and not targetable. If I moved the mouse over him, it still had the hidden power displayed as being active.
2) In another PVP match, the opponent used Black Fog and then El Toro attacked while hidden. He had a successful hit. After this, when it was my turn, El Toro was still hidden and not targetable. If I moved the mouse over him, it still said "Black Fog"