Adam Rutherford and his guests at the Hay Festival, neurologist Dr Suzanne O'Sullivan, acoustic engineer Professor Trevor Cox and science writer Dr Philip Ball discuss what scientists learn when things go wrong. Suzanne O'Sullivan, author of Brainstorm, talks about how she helps her patients with strange and unusual forms of epilepsy; Trevor Cox, whose new book is called Now You're Talking, describes cases where our voices change, such as stammering and foreign language syndrome; and Philip Ball, who is part of Created out of Mind, a Wellcome funded project about dementia and the arts, explores what happens when our brains age.