I believe it was in the late 80's, when Johnny Carson was ill for some reason, that they brought in various people to host the show for him while he was gone. And since these people were temporarily replacing Johnny as hosts of his late night TV show, they became guest hosts, which, of course, is a contradiction in terms. That, however, is only a single oxymoron.
When Johnny became more seriously ill, and was out of commission for a while, they brought in someone else, Garry Shandling, I believe, to replace him regularly. And Gary Shandling became the permanent guest host of the Tonight Show.
This formed a very rare linguistic occurence, the triple oxymoron, a phrase in which three words are all contradicting each other. Being someone's guest, is, after all, a temporary occurence, and being someone's host likewise. And, of course, since these terms contradict each other, it leads to a three way contradiction that would be nonsensical, besides of course, we all know what it means.