I do not believe in the supernatural, magic, ghosts or anything like that. However, I can be very superstitious about tempting fate and won't make jokes or flippant remarks that could be interpretted as such.
For example, my partner made a dark joke about how she'd rather have cancer than such and such. I begged her not to say such things, not because the thought of her having cancer upset me (although it did), but because it feels as if saying stuff like that could make it happen.
I have something that I would call a superstition. Sometimes, when I'm preparing for something (like going somewhere, preparing for a test, etc) and I have a weird thought about how something could go wrong, I have to make sure at least 3 times that I have covered against said possibility. So whether it be taking a particular thing with me, or like locking all my windows, or having my phone charged, or whatever, I make sure 3 or more times that I have done what needs to be done.
Surprisingly when I have overrode that feeling, even if I made sure once, I have 100% missed it somehow later on. So I just don't take chances now.