A Monk Reborn in a Lower Realm for Breaking a Leaf Where does the story appear about a monk who was born in a lower realm for breaking a leaf of a tree? 

Also there are some related stories (use generally alongside the above) where small transgressions leading to disproportionately large karmic results. What are they and where can I find references?
It's in the commentary to verse 182 of the Dhammapada:


  On seeing the Buddha, Erakapatta related to the Buddha how he had been a bhikkhu during the time of Kassapa Buddha, how he had accidentally caused a grass blade to be broken off while travelling in a boat, and how he had worried over that little offence for having failed to do the act of exoneration as prescribed, and finally how he was reborn as a naga.