desacordar

A specialized, mostly musical verb meaning to put an instrument out of tune — desacordar la guitarra. Older or literary Spanish also used the reflexive desacordarse to mean "to forget, to lose track of something," but that sense is now archaic and essentially unused.

• La humedad desacordó las cuerdas del violín. — The humidity threw the violin strings out of tune.
• No desacuerdes el piano antes del concierto. — Don't put the piano out of tune before the concert.
• En textos antiguos, desacordarse podía significar olvidar algo. — In old texts, desacordarse could mean to forget something.

Confusing desacordar with the far more common desafinar, which also means "to be/go out of tune" but is used more broadly, including for singing and playing badly, not just the state of an instrument's tuning.

Often confused with afinar.