Doesn't mean anything. The Arcing Smash miss was against a Feral Druid with Predatory Instincts - Spell - World of Warcraft. The only other miss I have seen so far was against a Shadow Fiend which have a similar passive ability. I have yet to see an Arcing Smash miss on a target that does not have passive AoE avoidance.
Which, if you really want, means taking Enveloping Shadows in the rogue-tank build. I'm fairly sure you can do that without losing anything essentially. That said, I haven't rogue tanked in ages cause the GM won't let me tank Shaz or Illidan. He's a killjoy in that regard.
I thought that it would be worth making a post about this fight as I feel there is quite a lot to discuss about it. We just killed it tonight after around 7 hours of attempts on it in total but I felt that it was more down to luck rather then mastering the encounter.
There is a little of both luck and strategy involved. 7 Hours of attempts is not uncommon in terms of tries.
From my raid experience is was always down to people making "those" mistakes and not fully focusing on what to do at certain phases.
It is one of those encounters that requires alot of raid focus and even still then it can be a little frustrating but stick it out and eventually everyone will be on the same page.