I admittedly stumbled upon a thief build that is particularly potent in WvW. It was mimicked after encountering a thief while in a small group of friends. The build revolves around using Cloak and Dagger, the number 5 skill with dagger/dagger. Essentially you can walk into groups of 30+ alone, and essentially cause hilarious unorthodox chaos by yourself. And if with only a bit of skill, you can escape unscathed. Essentially the skill involves a very short cast time, requires you to be in melee range, but places you into stealth if it lands. It costs a good chunk of your initiative, but if spec'ed properly, you should be able to regenerate health and initiative in stealth to continue indefinitely. You walk in, cloak and dagger the nearest person, backstab them or any other near victim, and repeat the process. ArenaNet has tried to counter this particular build by placing a short condition all stealth moves that prevent you from reentering stealth.
There is very little the enemies can do about it, and you can stay alive if you continuously move to a new target. At the moment, there are ways to counter it, but because you can do it in such large groups, there's almost always a plentiful supply of unsuspecting targets.
So if you see a thief charge towards a pack of thirty with that crazy look in their eye, they might not be as stuipd as you first think.