false note

Short definition from Wiktionary

Noun

1. 1
   An incorrect note which is sung or played in a musical performance.
2. 2
   In a remark or narrative, an indication (as discerned by the listener or reader) of untruth, insincerity, or inconsistency.
3. 3
   In a non-verbal display or presentation, an indication of incongruity or inappropriateness.