What happens if you put metal strings on a classical guitar?

You should never put metal strings on a classical guitar because the higher string tension can severely damage the instrument.

The lighter bracing and thinner top of the guitar can bend easily if not break at all.

The neck itself can bend, reducing or increasing the action of the guitar, resulting in an unplayable instrument.

If the guitar has a truss rod we can hope to recover the instrument, otherwise it could certainly be compromised