The theory that how you treat spiders affects how they treat you.
Jane: OMG I have so many spiders in my house these days. This morning I woke up with one in my hair and then a massive hairy one dropped into my cereal. I have to move.
Lesley: You must have bad spider karma. What did you do?
Jane: Umm... I used to use hairspray and a lighter on them. I didn't know any better.
Lesley: Harsh. You deserve it.
The little doubt in your mind that one day, a huge mutant spider will come after you and make you pay for all the spiders you have squished over the years. Manifests itself as a brief pang of guilt every time you swat an arachnid.
(*squish*) Hmmm... I have the bizarre notion that will come back to haunt me... must be the spider karma.