Are there any superstitions that you really believe?

Appears to have been a while since a similar question was asked.

I am not a superstitious person, if anything I’m a hardcore skeptic. In fact, I’m notorious for not believing in much of anything that can’t be proven beyond a shadow of a doubt.
However, the superstition that death comes in threes always gets me. I know that superstitions are nonsense, but whenever someone dies I will be on edge until I “figure out” the three, and I can’t relax until I’m certain it isn’t someone close to me.

Silly, but, it sticks with me.

Are there any myths, urban legends, superstitions or something similar that you have a hard time shaking?