The word “mercurial” seems to have been invented for Martha Argerich. Indeed, we could hardly write about her without it. One night this pianist can be world-beating: electric, insightful, inarguable. And the next night she can be music-beating: blunt, vulgar, ugly. Actually, she can be all of these things on the same night.

Friday night at Avery Fisher Hall, she played Prokofiev’s “Concerto No. 3,” with the UBS Verbier Festival Orchestra, Charles Dutoit, conducting. And she was, in fact, a mixed bag — both world-beating and music-beating.

Let’s start with the negative: She botched her opening figure, having kept her hands in her lap till the last second. Thereafter, she was sometimes rushed, mechanical, and, yes, blunt — blunt even for Prokofiev. She was guilty of harsh accents, and of slapping at the keyboard. She also went in for some sudden fortissimos — unwritten and incomprehensible ones. She tends to do this when she’s encountering technical difficulty (which is rare). It is a kind of cover. Furthermore, she experimented with strange rubatos, and strange tempo changes.

And, for all her volume and speed, she can be quite unexciting. You know those B-flat-major yelps, high up on the keyboard, near the end of this concerto? They ought to be thrilling; and, on this occasion, they had nothing. They were merely perfunctory.

But now for the positive: Ms. Argerich exhibited some of the catlike playing for which she is famous — tense, nimble, suspenseful. She imparted a degree of nuttiness, which Prokofiev needs. The opening of the middle movement was beautiful — absolutely beautiful. She did some exquisite trilling. And, no matter what her mood, no matter how she’s faring, you want to know what she’ll do next. This is an enviable trait in a musician.

The orchestra, under Mr. Dutoit, played admirably, and the cello section might be singled out: It sang beautifully in the last movement, and it sang together. After the concerto, Ms. Argerich sat down for an encore — a most strange encore. In rapid succession, without a pause, she played two pieces from Schumann’s “Scenes from Childhood” — and she played them out of order. First came “Catch Me,” as we know it in English; and then came “From Foreign Lands and People.” The first piece, she played with due impetuosity. And the second piece was perfectly lovely: gently rocking.

It’s funny about senior musicians and their repertoires (and remember that Ms. Argerich, incredibly, is now 66). They often narrow down to a handful of pieces; this is true even if they once played everything from A to Z. For the last several years, Ms. Argerich has played the Prokofiev Third a lot — a whole lot. And she has often done so in the company of Mr. Dutoit, to whom she was once married, long ago.

The two of them are touring America and Europe with the Verbier Festival Orchestra — 10 dates, with an unvarying program. Verbier is a ski resort in southwestern Switzerland, and, since 1994, they’ve had this music festival. New York has had a good season for festival orchestras from Switzerland: In October, Carnegie Hall hosted the Lucerne Festival Orchestra.

The Verbier ensemble calls itself a “training orchestra,” and it is composed of musicians from ages 17 to 29. They come from almost 30 different countries. Before the second half of Friday night’s concert, a flutist from New York introduced them all, by country — from Armenia to America. For some reason, Bulgaria got a big hand.

What Mr. Dutoit conducted was Berlioz’s “Symphonie fantastique,” that groundbreaking work from 1830. Mr. Dutoit has lived with this work for a long time; he knows it intimately. And on this evening he was at his absolute best. He was elegant, always elegant, but without sacrificing Berlioz’s wizardry. He was tidy and tasteful — yet Berlioz’s action was never cheated. Indeed, it was enhanced. For one thing, Mr. Dutoit’s sense of rhythm was superb.

Just as the fourth movement — the March to the Scaffold — was beginning, a youngster in the audience started to cry. Quite right — this is spooky music. And Mr. Dutoit’s march proved both spooky and stylish. The final movement — Dream of a Witches’ Sabbath — was duly freaky, but still elegant. The rendering of this music would have been too polite for some; I am one who found it highly effective.

Throughout the symphony, the Verbier orchestra played with accuracy and unity, commitment and understanding. One did not have the feeling of listening to an amateur orchestra at all (and amateurs they ain’t). And they were wonderful in their encore, as was Mr. Dutoit. This was the Farandole from Bizet’s “Arlésienne” — controlled and measured, but exhilarating.