Finny's second accident occurs as he rushes out of the Assembly Room where Brinker has organized a mock trial for the purpose of investigating Finny's first accident.  Distraught over the realization that Gene caused the accident that crippled him, Finny falls down the marble stairs at the front of the First Academy Building.

Perhaps as a defense mechanism, or perhaps because he really doesn't remember, Finny has never delved too deeply into the details of what happened when he fell from the tree.  Brinker's "investigation" and Leper's testimony force him to face the fact that Gene, his best friend, most likely jounced the limb on which they both stood on purpose.  The result was to make Finny lose his balance and fall to the riverbank, breaking his leg and rendering him unfit for the war.  When Leper describes how he saw the two figures on the limb move like an engine in which "first one piston sinks, and then the other one sank and fell", Finny and everyone else present understands that "the one who moved first shook the other one's balance".  Unable to accept that his friend had wanted to do him harm, Finny hobbles from the Assembly Room proclaiming, "I don't care", rushes down the corridor, and falls down the stairs (Chapter 11).