No DOUBT the error is an honest misunderstanding and entirely unintentional. But the secretary of the Treasury, Donald T. Regan, keeps saying with great assurance that the Mondale tax plan would raise the average household's taxes by precisely $1,860 a year. In this space we have tried to point out, as tactfully as possible, that Mr. Regan is once again wrong.

An interviewer asked him about that number, and the criticism of it, a few days ago on the "McNeill-Lehrer Report." Bristling, Mr. Regan said: "We're sticking by what we said. And, by the way, The Washington Post has a great nerve. They came out in that same place yesterday and endorsed Walter Mondale, and now pontificate in the same column. I think that's politics on The Post's part."

Politics? What a shocking thought. Any newspaper editoralist would be deeply wounded to be suspected of committing politics, or taking any interest in the subject whatever, in the delicate circumstances of a presidential election campaign. Setting politics aside and turning to arithmetic, Secretary Regan calculates that $1,860 figure, he explained, by adding up the costs of the Mondale program, as estimated by various Republican candidates, their press secretaries, the Treasury Department and other experts of similarly detached and nonpartisan standing. He then divided that figure by the number of taxpayers. If his method seems inadequate, you might want to note President Reagan's horoscope, as it appeared in Thursday's paper: "You win moral victory and gain in monetary sense." Any gain in monetary sense in this administration will be welcome.

Looking ahead, our own horoscope for Nov. 6 says: Watch out for former Wall Street figures who tell you things that strain credulity to the bursting point. But Mr. Regan is right about one thing. He reiterates that the administration has no plan to raise taxes. The administration seems to have no plans at all that reach beyond breakfast time next Wednesday. As for that huge budget deficit, Secretary Regan is following the well-established Micawber policy and trusting that something will turn up. Something, that is, other than the inflation rate.