Spilling water behind the person who has a very important event such as a job interview or is traveling will bring good luck. It is done for a happy ending. I find this custom very random in that throwing water does not seem to have much of an effect rather than get the hand wet but my Serbian stepmother told me throwing the water behind you is like replenishing the Earth and giving back to what has given you life so that it is good karma and the Earth will repay you.