Queued into 2s on my bad alt rdruid for some relaxing games with a family member's ret paladin and encountered an unholy DK (mcbad-tichondrius US) who could not take damage. We didn't notice until after we stomped his terribad rogue pal (Gitshrekt-Mal'Ganis US), but he didn't take even 1 damage all game. Even his pets, save the shambling horror, were invincible. He only left after it was clear that he wasn't going to win anytime soon (75% dampening before he gave up) and was likely to enjoy a free, six month vacation from WoW.
Assassination rogue, in its current iteration, is designed for players who want to achieve decently high rating without having to worry about the minutiae of PVP gameplay, to wit, situational awareness, dynamic responsiveness, or even keybindings.