Careful. If you cancel on a comedian, you get a free roast in your absence. Only "Jimmy Kimmel Live" and "Conan" are airing new shows this week, Fallon and the other late-night guys are on repeats, and both Jimmy Kimmel and Conan O'Brien saw their main Tuesday night guests cancel at the last minute.

So the hosts did what all decent comedians would do -- they burned the guests behind their backs.

Kimmel is in Brooklyn this week, and even though Donald Trump's team gave him about a day's notice to cancel, it's still a huge blow to lose Trump as a guest. Kimmel told the audience they're still confused as to what happened. "His people called late last night and canceled on us," said Kimmel (via The Hollywood Reporter). "They were cryptic as to why he canceled — they said he has a major political commitment. But he did want me to relay the message to you that if he had been here, he would have been great. ... I'm dying to find out what this major political commitment was. Usually, it means he had to go on CNN and call someone an idiot or something."

Kimmel also jokingly asked, "Why did he cancel? We told him there were cameras here, right?" Trump is supposed to be on the show at a later date, but Kimmel told his October 20 audience not to worry, "Tonight, we're going to give everyone in the audience a basketball dipped in cologne, so you can fully experience what it would have been like had Donald Trump been here."

Nice. Tracy Morgan stepped in for Trump and he hung out with Jimmy at the zoo.

Conan and Andy Richter also had fun talking about how "Heroes Reborn" but he canceled that day. Why? Conan said it's very unusual for a guest to cancel the day of the show. Zachary didn't have a political or other commitment, he just didn't get on the plane -- that they helpfully set up for him, Conan added. The actor had a flight that morning from Toronto that he was supposed to get on, and then he just didn't make it.

So Conan asked Andy, in his experience, if he misses a morning flight, what does that normally mean for him? Andy: "I drank too much and overslept." Conan: "Yes!" Conan said he was curious, so he asked others in the studio (including a stray dog) the same question and got the same answer. So basically they're just assuming Zachary Levi got drunk and overslept.

Conan: "Well it's too bad, because since I began on television in 1993, I've been planning all this time to honor my show's 10,267th guest. See, that's a sacred number in both Aztec and Irish culture. So this was going to be a huge deal." Andy said he's been talking about it for years. Conan said they had some amazing prizes lined up for Zachary, including a multi-million villa off the Amalfi Coast. Since Gabrielle Union actually did show up, she got all the prizes ... except the stray dog.

Check it out:

Basically, both hosts made lemonade out of the lemons they were dealt. And at least Zachary had the grace to make fun of himself for his boo-boo.

Here's Zachary apologizing and promoting Conan's show:

Here's a roundup of what did happen last night on the late night circuit, thanks to the guests that bothered to show up.

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