There are two common impressions of Mozart’s music. The principal one is of what could be called “Musicbox Mozart,” the graceful, elegant sort of piece epitomized by “Eine Kleine Nachtmusik.” In recent years a second impression has developed, courtesy of the play (and then movie) Amadeus. This could be called “Romantic” Mozart, dark-hued, even demonic. (It’s telling that the “Romantic” nineteenth century already recognized, and was drawn to, this side of Mozart, as represented by music like the Overture to Don Giovanni, the outer movements of the D minor Piano Concerto K. 466, or certain sections of the Requiem.)
But there are still other facets to Mozart. One of them is his “advanced” side, in which he pushes the envelope of convention, especially in the sphere of harmony, where his music could be daringly, even shockingly dissonant. His K. 465 string quartet is in fact nicknamed “The Dissonant,” owing to a wrending moment in the slow introduction to its first movement. But a spot in a different Mozart quartet is even stranger (and much less well known for its strangeness). The minuet in his last quartet, K.590 (at 14:56 in this clip) starts sweetly, even innocently, but in its second half (beginning at 15:30, and then repeated) the harmonic stress-level rises to the point of going over the top.
Towards the end of his life, Schubert said that he was imagining harmonies that had never been heard before. It wouldn’t be surprising if some of them resembled Mozart’s harmonies in this minuet, which reside at an outer limit of the classical musical language.