user295: 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?

user254: 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.


295: <p>Where does the story appear about a monk who was born in a lower realm for breaking a leaf of a tree? </p>

<p>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?</p>
 254: <p>It's in the commentary to <a href="http://www.tipitaka.net/tipitaka/dhp/verseload.php?verse=182" rel="nofollow noreferrer">verse 182 of the Dhammapada</a>:</p>

<blockquote>
  <p>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.</p>
</blockquote>
