For example, yesterday was the Feast of Corpus Christi. The Office and Mass for the feast were written by St. Thomas Aquinas. The lyrics of well known hymns like Pange Lingua (of which Tantum Ergo is a subset) or O Sacrum Convivium come from this work of his.
I think it was St. Bonaventure (I'm sure a FReeper will correct me if I'm mistaken) who was also asked to write the Office for the Feast of Corpus Christi. He and St. Thomas both met with the Pope. After hearing St. Thomas' composition, St. Bonaventure tore his up.
I've seen this "incident" referred to on a few websites. One refers to it as a "pious legend," while another treats it as fact.