My (probably incomplete) list:
* blowing out birthday candles
* returning the clasp of a necklace to behind one's neck
* passing a truck carrying a load of hay
* falling star/first star of the night
* eyelash (blown off the fingertip)
* when all the digits on a clock are the same number
* blowing the fuzz off a dandelion puff
* breaking a wishbone
* throwing a coin in a water fountain/wishing well
Others?