bum note

noun2

1. •
   (slang) A note that is played incorrectly.
2. •
   (slang, figurative) A social or conversational mistake; an awkward or inappropriate action.

   • 'I'm going for a pint.' Declan paused on the way out. 'You really think Ethan's that good? There could be money in it?' He realised he had struck a bum note. — 2013