Pentobarbital is common execution drug. So why was Christa Pike's execution botched?

When it comes to executing death row inmates, pentobarbital is the drug of choice for more than a dozen states and the federal government.

It has been used to kill more than 100 murderers in the U.S. for more than 15 years. So how did Christa Pike survive her execution in Tennessee after a doctor injected her with two doses of pentobarbital on Sept. 30?