Sir Humphrey has one near the end of "The Moral Dimension" — Hacker tells Sir Humphrey and Bernard that he intends to blow the lid on the corruption involved in a massive contract between the UK and the Arab state of Qumran, along with telling the press about how Bernard forged a valuation certificate so that Annie could keep a rosewater jar that she wouldn't otherwise have been allowed to hold onto. Even though it will likely destroy both the contract and the UK's relationship with Qumran, Hacker goes on a speech about how it's the moral thing to do, and therefore the right thing. Sir Humphrey seemingly agrees with Hacker... and then matter-of-factly asks Hacker which one of them will tell the press about the "Communication Room" (read: Alcohol Smuggling Room) that Hacker set up in the Qumran royal palace earlier in the episode, completely demolishing Hacker's position in the blink of an eye.
Then becomes a CMOA for Hacker, as he has to completely make up a plausible story to tell a journalist with no time to prepare. He successfully manages to answer or deflect all the questions, to the point that even Humphrey says it was "superb". Now, if only he could display that level of savvy and cunning at other times...
In a meta-sense; you know all that Sesquipedalian Loquaciousness that Sir Humphrey was constantly doing? Those long, convoluted, complex speeches? Nigel Hawthorne memorised them. And could still repeat some of them several years later.
Another meta-moment: The opening title sequence was not just hand-drawn by Gerald Scarfe, but they were not animated with separate drawings. Scarfe had to draw with a camera in his other hand and take two pictures in-between brush strokes, and these pictures became individual frames in the animation. That is extremely difficult to do with any sort of accuracy, much less the kind of accuracy you see in the title sequence.
Hacker's decisive victory over Sir Humphrey in the Yes, Prime Minister episode "The Key" is the culmination of a brilliant piece of farce and amazing acting from both Paul Eddington and Nigel Hawthorne.
Hacker's first real, unqualified, triumph over Humphrey was in "The Skeleton in the Cupboard" when Hacker realizes he finally has leverage to get Humphrey to do the right thing and mercilessly exploits it with undisguised glee.