The spawn dance
There are many players out there that subscribe to the particular belief *cough* Maelis *cough* that a special dance will help mobs spawn faster. I have never myself attempted to use a spawn dance, but I have found that I tend to run in a clockwise direction when I am farming because somehow the mobs seem to spawn faster. Or perhaps it's just because I can't turn left.
This superstition is so common I hardly ever not seen it when grouping. We tend to somehow believe that the rolls are not random, that we can somehow bring good luck or bad when rolling on loot. I know for a period of weeks I lost every roll in an instance, and was convinced I was cursed. I have also been known to wait until someone else rolls on a chest or BoP item so that hopefully their low rolls will boost mine. I haven't gone this far though:
I've seen people roll their dice repeatedly when joining a raid, stopping just after a terrible roll because they were 'getting the bad rolls out'. They weren't kidding.
The "don't do that or else" principle
There are several examples of imagined risky behaviors in the game, those things we avoid doing because we fear a catastrophic result despite not having any hard evidence. "Don't sunder the Beast in UBRS or he will AoE nonstop" or "Don't use Curse of Weakness on Gandling or he will start teleporting people a lot faster." We have no proof, none of us have seen it in the code of the game, but we assume these little superstitions to try to get an edge on the fights. I know for a while back before the expansion I wouldn't use fire spells on the fire mages in LBRS for fear of retaliation.
Try as I might, I simply can't shake the superstitions I follow. I realize they are silly (after all how can my holding my breath affect a random number generator?) but I can't help myself. I suppose it's my little way of trying to control situations I simply have no power over. Or perhaps it is my way of connecting with the rest of humanity as we play this game we call World of Warcraft.