Andre Watts, pianist, made mistakes last night at the Kennedy Center Concert Hall. As the soloist in two of the Beethoven concertos explored in this year's National Symphony Orchestra Beethoven Festival, Watts made big mistakes, audible mistakes, and more than a few of them. And thank God for it.

Watts is the Andrea del Sarto of soloists, flawless and sometimes cold but always professional and reliable. He was an obvious choice given the logistical demands of the NSO's challenging plan: to perform all of Beethoven's major works for solo instrument and orchestra between now and Sept. 18. Watts is the rare pianist who would undertake two concertos--by rough estimate, about 25 percent of the festival's major works--in one evening (though pianist Garrick Ohlsson will attempt the same feat on Sept. 17).

No one ever ruined a Beethoven piano concerto by giving away the ending. They are, in many ways, conservative works, three-act dramas in which the characters may change but the central conflict--the one vs. the many--never does. The choreography of the pugilists may take unexpected turns, but the results are preordained: victory, or at least reconciliation.

Making them live means introducing surprise into ritual. Last night, that surprise came in the form of small blunders--insignificant really--but just unsettling enough to make the music sparkle.

The divide between audience and critical reaction couldn't be wider for Watts. He has been before the public more than 30 years, and long since graduated from celebrated prodigy to a professional--some would say omnipresent--denizen of the world's greatest concert halls. His big and effortless playing has attracted uncommon critical venom. It is for many too effortless. As a concerto soloist, Watts's reliability can be maddening. In these divine dialogues, with their successive moments of rancor, rapprochement and introspection, Watts is the speaker who makes all the salient points, hits all the relevant details. But what did he say?

Last night the unflappable pianist sounded more than a little flapped, dropping notes and arriving at prominent landmarks sometimes too soon, sometimes too late. But the musical results, as if in compensation, were fascinating. There were moments, especially his ferocious yet childlike entry into the last movement of the first concerto, when one heard what must have so appealed to the pianist's earliest fans: the prodigy who is just a little too good at it all, pushing limits in an innocent way.

The same Watts returned for long passages of the two slow movements, reveries in which he turned familiar phrases with unexpected pliancy. And despite problems in the opening pages of the Piano Concerto No. 5 ("Emperor"), he was back for its swaggering conclusion, producing the large and domineering sound that works, at times like these, marvelously well.

Watts was fortunate to have the NSO behind him. Yes, it would have been nice if they had entered and exited together a bit more often, but if some of the macroscopic details were flubbed, there were many small moments. Music Director Leonard Slatkin brought forth sunny sounds from the strings, giving uncommon and benevolent attention to secondary lines. The woodwinds produced an even, controlled sound reminiscent (when used together) of an organ stop. Individual players seemed to have dipped into some of the soloist's adrenaline, increasing the vigor of the collective sound.

The official opening of the Beethoven Festival actually came Wednesday night when the members of the Ying Quartet examined three major quartets from different periods of Beethoven's output. The Yings offered a stark contrast to the NSO. It was a quiet evening, with better-rehearsed music and, in the Op. 131 quartet, perhaps the most musically substantial work of the festival. The Yings, four siblings age 29 to 35, have grown up before the public. In 1992, they participated in the National Endowment for the Arts Rural Residencies Initiative, which placed them in tiny Jesup, Iowa (pop. 2,000). They took their role seriously, with humility, dedication and excitement--qualities they brought to the performance Wednesday evening.

The program included the Quartet No. 5 in A, Op. 18, which burbles happily in the fashion of Beethoven's predecessors, Haydn and Mozart; the "Harp" Quartet, Op. 74, in E-flat, a product of the great efflorescence of Beethoven's middle years; and the emotionally searing, sui generis Op. 131. If the early quartet brought forth some undigested sounds--insufficient attention to basics like intonation and articulation--it seemed, in perspective, as if it were music they were willing to pass by quickly. The "Harp," however, found them playing at peak, and the Op. 131 quartet pushed them just beyond it.

The Yings played that masterpiece exactly the way a group of their age should: with virtuoso exhilaration, touches of humor (sweet, not mordant) and recklessness. They made the quartet a succession of perhaps disconnected emotions, ending with tremendous fury. But if they were disconnected emotions, each was convincing in its own right.

CAPTION: Soloist Andre Watts put the element of surprise back into well-known Beethoven works at last night's National Symphony Orchestra concert.