If you tell us what beer you find it in, we could all go and find out. Those of us who'll deliberately buy bad beer, that is.
Mind you, my missus and me both use "musty" to describe an off flavour we sometimes get in yellow lager, but we [i:239li0r5]never find it in the same yellow lager as each other[/i:239li0r5]. I think some of taste is just genetic.