General Kelley: “Please don’t take anything the President said as asking you to do something illegal or unethical”
I'd like to point out that again this was, in some people's view, H. R. Haldeman's primary job.
Nixon was prone to his paranoid crazy and utterly illegal ideas that he came up with on the spot and would order people to do. But those close to the President knew he was just ranting and often would never even remember the conversation let alone his orders. Haldeman, and Ehrlichman (and occasionally Pat Buchanan), adopted a policy of pushing back at Nixon to see if he was in the state of mind to drop it and if he was locked in on the idea just saying they'd take care of it, then doing nothing about it or even mentioning it to anyone. That solved 90% of the problems.
This gatekeeper system had only a couple flaws, one was that Kissinger did everything he could to circumvent it and even though Nixon hated him and distrusted him to the point of that being part of the entire reason the Oval Office switched from a manual to an automatic system of recording (also Nixon never remembered to turn it on...this was all after him ordering the entire thing ripped out upon taking office) ol Henry would walk Nixon into musing and then use it to go do his own thing claiming authority. (Ford, Rumsfeld and Cheney upon taking office as President/Chief of Staff/Deputy immediately identified Kissinger's behavior as the biggest problem they had internally and when they demanded Kissinger show up on time he threatened to resign (this was his usual strategy) and TELL ALL but Ford called his bluff... it's also why Kissinger started wandering around the world trying to solve random civil wars as they were happy to have him out of Washington.) The other flaw was that if it broke down, and there's literally no explanation that I've even seen made as to how some of these happened, ideas like breaking into Ellsberg's medical records could get past the gate. I suspect that Watergate for example, was far more of a campaign operation which was borderline unchecked and run by people like Liddy, which was why Nixon and Hadleman didn't know about it. And Hadleman and Ehrlichman must have uncharacteristically bungled cleaning up the mess. Or it was just snowballing too fast for the era. Or like Nixon, they were more afraid of the other stuff that was on the tapes.
