How did US inmate Christa Pike survive two 'lethal' injections?

Christa Pike, who has spent more than three decades on death row in Tennessee, survived two doses of what was supposed to be a lethal injection and was taken to hospital. A court-ordered pause was lifted by the US Supreme Court before the attempt, and the state's governor has halted executions for the rest of the year and ordered a review. So how can an attempted execution go so wrong? Be aware that this story includes disturbing details of a crime and how the death penalty is administered.