The Washington Post's fact-checker gave Spicer's statement four Pinocchios at the time, the worst rating, reserved for "whoppers."
There was also the time he criticized Syrian President Bashar al-Assad's use of chemical weapons, and ended up downplaying the horrors of the Holocaust. Spicer said that even Adolf Hitler did not sink to that level of warfare and "was not using the gas on his own people in the same way that Assad is doing," despite Hitler's use of gas chambers to kill millions of Jews and others.
"I screwed that up royally," Spicer told Cupp, adding that "when I screwed up, yeah, it felt really bad."
"You realise that you're tarnishing your personal reputation, your family's reputation, your friends who like and support you, some of your colleagues and ultimately . . . this administration and the American people."
Indeed, Spicer has been asked numerous times in recent months to open up about his missteps and regrets from his time in the White House. Jimmy Kimmel mocked him to his face about the inauguration crowd size remarks. He even poked fun at his own claims in a controversial cameo at the Emmys.
"I honestly went out every day to do the best job I could for the president of the United States who gave me an unbelievable honour," he said.
Asked by Cupp on Thursday if he ever blatantly lied on behalf of the president, Spicer said he had not.
"There's no question that sometimes . . . information changed as facts became available to us . . .," he said, but added that "at no time did I go up and say something that was . . . demonstrably false."
Spicer argued that "there was an intensity and a scrutiny that has never been seen before in a White House."
He said that while the role of White House press secretary was his dream job, he quickly realised his boss would be unlike any of his predecessors' bosses. He needed to adapt to the style of an unconventional president — a task that he admitted was "absolutely" difficult.
Spicer, whose tenure was marked by a contentious relationship with the press, resigned in July after clashing with Trump over the appointment of Anthony Scaramucci as communications director.
He has since announced that he will be releasing a book in the summer to "set the record straight" about his time with the Trump administration. The book, titled The Briefing, will be released on July 23.