What is an example of a common, yet sinister, superstition that people blindly adhere to with casual indifference?

One common superstition dictates that we should not walk under a ladder or else something bad will befall us. Another such superstition surrounds black cats; if one allows a black cat to cross one's path, then bad luck will also befall that person. Yet another superstition decrees that if we break a mirror, it will result in seven full years of terrible luck. A fourth says that if we spill salt, we must throw a pinch of it backwards over our shoulder in order to avoid bad luck. Opening an umbrella inside is said to be unlucky, as are Fridays that fall on the thirteenth of the month.
The idea that something totally random, such as breaking a mirror or spilling salt without throwing some over one's shoulder, could actually bring a person bad luck—bad luck in the form of injury or illness, financial strain or job stress, really any area of one's life—is pretty sinister. Accidents happen, right? Things get spilled and broken, we cannot control the walking patterns of cats, and maybe there's just no other way around the ladder but to go under it. To suggest that one could experience some trauma in one's life simply due to one of these reasons is menacing and fear-inducing indeed.
How about the idea of keeping a rabbit's foot on your person in order to attract good luck? It's literally the foot of a dead animal—I'd say that's pretty sinister. It certainly isn't good luck for the rabbit!