President Trump's recently-proposed budget contains a massive accounting error: it predicts a $2 trillion increase in federal revenue--to be used in what the president's been calling the "biggest tax cut in history"--but it then uses that same $2 trillion to balance the budget.

White House budget director Mick Mulvaney (a founding member of the notoriously conservative House Freedom Caucus) saw no problem in the apparent miscalculation. He said it was done "on purpose."

"I'm aware of the criticisms and would simply come back and say there's other places where we were probably overly conservative in our accounting," he said. "We stand by the numbers."

The Treasury Secretary Steve Mnuchin reassured critics that the budget is "a preliminary document that will be refined."

But Larry Summers, who served as Treasury Secretary from 1999-2001, wrote on his blog, "It appears to be the most egregious accounting error in a presidential budget in the nearly 40 years I have been tracking them."