3 on a match

Superstition. Use the same match to light three things - candles, cigarettes, whatever - and you summon a ghost.

This Superstition comes from the rather valid problem of lighting cigarettes in the trenches. If you leave a flame burning long enough to light three cigarettes, you have left enough time for a sniper to shoot at least one of you.
Which, in the end, makes that whole lung cancer thing kind of moot.