John Oliver has been one of the most outspoken voices against Donald Trump this election, but he admitted he made a big mistake in the past about the presidential candidate.

"It is frankly hard to believe that there was a time when people thought that a Trump candidacy would be funny," Oliver said Sunday during the opening of his HBO show "Last Week Tonight." "But there was such a time."

Oliver then showed a clip of him guest hosting "The Daily Show" from three years ago that had the comedian comment on reports that Trump was considering a run in 2016.

And, well, the clip showed that Oliver actually egged on Trump to run.

"Do it," Oliver said to Trump in the clip. "I will personally write you a campaign check now on behalf of this country, which does not want you to be president, but which badly wants you to run."

The show then went back to present day Oliver who was hit with laughs and jeers from the audience.

"Now, in my defense, I have no defense for that and was hoping to think of one before finishing this sentence, which oh s**t it's over," he said.

It wasn't the only bad prediction Oliver made about 2016.

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"I want to believe that America will reject Donald Trump. That our innate sense of decency will kick in like some sort of moral auto-pilot," Oliver added. "But I have been spectacularly wrong before."

He then showed another clip of "The Daily Show" from eight years ago that had Oliver saying that the "Chicago Cubs will never, ever win the World Series." Something that the Cubs finally did after 108 years last week.

"I'm an idiot," Oliver said. "But that clearly shows no outcome is certain, so if you are thinking you don't have to show up to vote on Tuesday because there is no way the impossible could happen, take it from somebody who has learned from painful experience... you are wrong about that."

HBO, like CNN, is owned by Time Warner (TWX).