The Bodhisatta was once a very skilled acrobat and travelled about with his teacher who knew the dance of the four javelins. One day the teacher, in a drunken fit of boasting, announced that he would do the dance of the five javelins - which he did not know - and insisted on doing it against the advice of the Bodhisatta. The result was that the boaster was impaled on the fifth javelin (J.i.430f).
For the introductory story see the Gijjha Jātaka (No.42).