What hasn't happened  in the past is a former FBl agent that is charged with colluding with the wise guys in a murder. That doesn't happen—and that's shocking. 

我认为，原句抄写有误， a former FBl agent 和 that，被颠倒了位置。句子应该是这样：

What hasn't happened  in the past is that a former FBl agent is charged with colluding with the wise guys in a murder. That doesn't happen—and that's shocking. 

【翻译】过去没有发生过的事情是，一名前联邦调查局特工被控与智者串通谋杀。这是不可能的---这太令人震惊了。

What hasn't happened  in the past is that a former FBl agent is charged with colluding with the wise guys in a murder.

为了强调 What hasn't happened 这个表语成分，本句运用修辞法，而让“主表倒装”，先说表语What hasn't happened，再说主语that 从句。