The Bodhisatta was once a farmer. A traveling salesman used a donkey to carry his wares, and when it was not needed he draped a lion skin over it and set it loose in barley and rice fields. People seeing it were normally too afraid to shoe the lion away, so they just let it eat their crops. When he came to the Bodhisatta’s village, a watchmen gave the alarm about a lion and all the villagers came out shouting, blowing conchs, and beating drums. The donkey got scared and let out a hee-haw. The Bodhisatta then realized it was a donkey in disguise and when he announced this, the villagers ran up and clubbed it to death. When the salesman came back, he lamented that his donkey was too stupid to keep its mouth shut and then went on his way.
In the Lifetime of the Buddha
The donkey was an earlier birth of a dimwitted disciple of the Buddha who was completely unaware how stupid he was. He made it known to other disciples that someday he wanted to recite scripture at assembly, and he was invited to do it. That evening he took his seat in the jeweled pavilion in front of the others, and just as he was to begin speaking he grew nervous and ashamed. He got the first verse out, but could not remember the second, so he left and went to be alone in his quarters.
Later, when the Buddha heard some disciples discussing the incident, he told them this story so they knew that this empty disciple had also betrayed himself by speaking in the past.