It could possibly be a sport, which is a genetic mutation (which plant breeders sometimes use to create new cultivars). Or, it could be a reversion--when a stem produces characteristics of the parent plant instead; some mutations (such as variegation in certain cases) are not stable, and will sometimes revert back to a parent characteristic. Another possibiity is that the change was caused by a virus or environmental condition. Unless you are a knowledgeable plant breeder and this is a mutation, then it is probably best to prune it out--especially if it is caused by a virus.
It could possibly be a sport, which is a genetic mutation (which plant breeders sometimes use to create new cultivars). Or, it could be a reversion--when a stem produces characteristics of the parent plant instead; some mutations (such as variegation in certain cases) are not stable, and will sometimes revert back to a parent characteristic. Another possibiity is that the change was caused by a virus or environmental condition. Unless you are a knowledgeable plant breeder and this is a mutation, then it is probably best to prune it out--especially if it is caused by a virus.