false note

noun3

1. •
   An incorrect note which is sung or played in a musical performance.

   • Sometimes he even thrums a little on the piano, and takes a part in a trio, in which his voice can generally be distinguished by a certain quavering tone, and an occasional false note. — 1819
2. •
   Of a remark or narrative, an indication (as discerned by the listener or reader) of untruth, insincerity, or inconsistency.

   • [H]is words were grave, his manner was earnest, and his speech came from the fulness of his heart. If there had been a false note, a false look, Harry would have detected both, […] — 1900
3. •
   Of a non-verbal display or presentation, an indication of incongruity or inappropriateness.

   • The only false note in an otherwise classic and elegant collection was the gratuitous flash of black fishnet stockings and suspenders under otherwise unremarkable paisley and floral shirts. — 2009