I have seen it quoted in this manner: "If there are two or more ways to do something and one of those results in a catastrophe, then someone will do it that way." It is more a psychological law than a superstition then.
Commonly, a situation where, if anything can gowrong, it will, but I wanted to add my examples.
You always get your blackjack one or two hands after you lower your bet.
If you take the phone with you outside, say to garden, it will never ring, but if you leave it inside there will be several messages on your voicemail.