I'm not actually a very superstitious person, but I do have a few. My one dice superstition is that the old dice I collected from my TSR box sets are now MY dice. No one else gets to touch them.
Back in the day, we just didn't have that many dice. KillingMachine, my cousin Ben, and I each had two sets from our Basic and Expert sets, an extra pair of d10s each from Star Frontiers (which usually stayed in the SF box), and whatever extra d6s we scrounged from old board games. And if one of us was at another's house and didn't bring our own, of course we shared our dice.
Now, though, I've got more than enough other dice that the old ones stay safely far from others in their own little gray dice bag. And I mostly use them when rolling up my Megadungeon encounters or monster hit dice or number appearing and all that, not much for actual gaming.
Anyway, last Saturday, for some reason I brought that set of old dice to the 4E game. And they were rolling quite poorly.
I wonder if my dice were trying to tell me something.
Just in case, I'll be bringing a different dice bag to the next game.