false note

A false note is something that seems wrong, insincere, or out of place in a given context.

The expression comes from music, where a false note is one that deviates from the correct pitch or doesn't belong in the harmony.

In figurative use, it describes moments when words, actions, or elements of a performance ring untrue: Her apology struck a false note with the audience.

The film's comic subplot was a false note in an otherwise serious drama.