A decade ago, archaeologists found two 3,000 year-old mummies buried on Scotland's South Uist island. The mummies had been preserved by immersion in a peat bog but not actually buried for another several hundred years. Turns out though that the skeletons aren't actually a single man and woman but rather bones from six people that someone painstakingly attached together. A mismatched jaw was one clue and DNA tests confirmed scientists' suspicions. From National Geographic:
A cynical theory, study author (and University of Manchester biomedical archaeologist Terry) Brown said, assumes that the Bronze Age people of Cladh Hallan were just eminently practical: "Maybe the head dropped off and they got another head to stick on."
Another possibility is that the merging was deliberate, to create a symbolic ancestor that literally embodied traits from multiple lineages.