The Buddha explained, "Thousand and thousands of kalpas ago, this man was born as a fly. He was sitting on a pile of cow dung when a sudden rush of water caught the cow dung, along with the fly, and sent them into the river. Downstream, someone had placed a prayer wheel in the water, and that cow dung and fly swirled around and around it. Because of that circumambulation, this man now has a seed to attain arhatship in the lifetime."
Cause and result are so subtle that only omniscient wisdom can perceive every detail. That is why we must be very careful that our actions are truly beneficial.
Reciting just one mantra, protecting the life of even one small bug, giving a small thing--we should not ignore such action by saying, "This is nothing; it makes no difference if I do it or not." Many small actions will gather and swell like the ocean. These are not merely Buddhist beliefs; these are the causes that create our world no matter what who we are. Our study and practice give us the opportunity to understand this and to be sincere with ourselves even in small things."
--from A Complete Guide to the Buddhist Path by Khenchen Konchog Gyaltshen