(CNN) Even on its best days, being the press secretary for President Donald Trump is a near-impossible job.

Thursday was not one of White House press secretary Sarah Sanders' best days.

The night before, she had learned that Trump had -- contrary to Sanders' past assertions -- reimbursed his personal attorney, Michael Cohen, for the $130,000 hush payment to porn star Stormy Daniels 11 days before the 2016 election.

How did she learn that troubling fact? The same way the rest of us did: By watching former New York Mayor Rudy Giuliani on Sean Hannity's show on the Fox News channel.

Talk about a gut punch.

Here's a quick refresher course. On March 7, Sanders was asked whether Trump knew anything about the payment to Daniels. Here's what she said: "Yeah, I've had conversations with the President about this. ... There was no knowledge of any payments from the President and he's denied all of these allegations."

It's not hard to see what happened there. Knowing she would be asked about the hush money, Sanders asked Trump. And he told her something that simply wasn't true.

Which brings us to Thursday, when Sanders was forced to face down the White House press corps after this public embarrassment. What explained the discrepancy between what she said on March 7 and what Giuliani said on Wednesday night? This exchange with CNN's Jim Acosta tells that story:

Acosta: "Why can't you just answer yes or no whether you were in the dark. I think it's a fairly simple question whether you just didn't have the information at the time."

Sanders: "I think it's a fairly simple answer that I've given you several times now. I gave you the best information that I had and I'm gonna continue to do my best to do that every single day."

In this instance, "the best information I had" is a euphemism for "the President of the United States didn't tell me the whole truth and then I went out and repeated it."

To the extent Sanders retained credibility among the White House press corps, she lost it with that answer. Admitting that you misled the press because the President misled you is tantamount to taking your credibility, pouring gasoline on it and then setting it on fire.