vinyl records are vulnerable to dust, heat warping, scuffs, and scratches. dust in the groove is usually heard as noise and may be ground into the vinyl by the passing stylus, causing lasting damage. a warp can cause a regular "wow" or fluctuation of musical pitch, and if substantial it can make a record physically unplayable. a scuff will be heard as a swishing sound. a scratch will create an audible tick or pop once each revolution when the stylus encounters it. a deep scratch can throw the stylus out of the groove; if it jumps to a place farther inward, part of the recording is skipped; if it jumps outward to a part of the groove it just finished playing, it can get stuck in an infinite loop, playing the same bit over and over until someone stops it. this last type of mishap, which in the era of brittle shellac records was more commonly caused by a crack, spawned the simile "like a broken record" to refer to annoying and seemingly endless repetition.