It must be resolved

Bach, a master of harmony and counterpoint, would not settle for imperfect sounds, no matter where he was.  Johann Reichardt recalled:

Johann Sebastian Bach once came into a large company while a musical amateur was sitting and improvising at a harpsichord.  The moment the latter became aware of the presence of the great master, he sprang up and left off with a dissonant chord.  Bach, who heard it, was so offended by this musical unpleasantness that he passed right by his host, who was coming to meet him, rushed to the harpsichord, resolved the dissonant chord, and made an appropriate cadence.  Only then did he approach the host and make him his bow of greeting.

Cited in: The New Bach Reader: A Life of Johann Sebastian Bach in Letters and Documents.  Edited by Hans T. David and Arthur Mendel.  Revised and expanded by Christoph Wolff. New York: Norton, 1998, p. 411.