Here's one: the adjective iūgis "continuous, persistent, perduring, everlasting", and iugīs, the ablative plural both of the adjective iugus "joined together, joined by means of a cross-member, yoked, and (figuratively) married", and of the noun iugum "a cross-member, a crossbeam, a yoke". Incidentally, iūgis and iugus/iugum derive from two completely different IE roots, which was not generally recognized until about 1994. Before that, either they were assumed to derive from the same root, or that iūgis derived from iugum (!), which consideration might make some type of (vague) sense semantically, but posed both morphological and analogical problems.