A long time ago I was at a pool party at my friend's house. After hours of playing in the sun without any proper protection, majority of us were sunburned. After we had all showered and eaten, we went in search of pain relief. There were some aloe plants in the yard, so me and 2 other friends went to cut some. When we returned, some of our friends scolded us saying that we shouldn't cut plants at night because it attracts the spirits. I had never heard of this before, and this was just the beginning of many superstitions that I have accumulated over the years.
At the new year, I've heard that you shouldn't sweep your house because you sweep all the good luck away. I think this is a Japanese superstition, but I am not entirely sure as I am not Japanese.
I try to refrain from whistling at night, because this is also said to attract spirits. I don't know the origin of this superstition.
Pointing at graveyards and gravestones is taboo. If you point, the spirit of the dead will grab onto your finger and not let go. I actually recall something like this in the movie "League of Their Own," but they crossed their fingers and held it over their heads.
I know that there are more, but I can't seem to think of them all at the moment. I invite you to add your own superstitions that you've grown up with or heard of!