Until the 13th century, authors equivocated when speaking of the unspeakable sin alluded to in the Life of St. Giles. Some authors continued to dodge the issue in the centuries to come, but others did not. They made shocking accusations against Charlemagne and committed them to writing. Perhaps the accusations themselves were invented in the 13th century, or perhaps such claims had long circulated awaiting the moment when authors finally mustered the courage to write them down. Be that as it may, authors offered two different identifications of Charlemagne's unspeakable sin. Some authors identified Charlemagne's unspeakable sin as incest. Specifically, they claimed that incestuous relations between Charlemagne and his sister, Gisela, had in turn resulted in Gisela giving birth to Roland, the hero of the Song of Roland.

The allegation of an incestuous relationship between Charlemagne and Gisela appears in the Karlamagnus Saga, a 13th century account of Charlemagne's life written in Norse. From there, the incest claim is then taken up by a number of different texts, especially French texts.

Other authors identified Charlemagne's unspeakable sin as necrophilia. That claim appears in a 14th century German poem about Charlemagne, "Karl Meinet," and the idea was then taken up in a number of 14th and 15th century German chronicles and treatises.

Specifically, someone had hexed Charlemagne by placing a charmed ring under the tongue of his dead wife. The ring caused Charlemagne to become infatuated with the wife and to continue the relations they had had while she was alive. When a bishop discovered the ring and removed it from the dead wife's mouth, Charlemagne became infatuated with the bishop. The bishop tossed the ring into a swamp, and Charlemagne became infatuated with the swamp, building a palace and dwelling there. To be clear, none of this is true...

One can only speculate as to why stories arose alleging that Charlemagne was guilty of incest or necrophilia, and why those stories gained a significant and distinguished audience. These stories did not emerge in or remain confined to a specific geographical milieu. They do not seem to have been concocted to achieve any specific political outcome. Perhaps they emerged primarily as a reaction against the overblown praise that Charlemagne received in other works. The bigger they come, the harder they fall.

Mating frogs may have been occasionally getting it wrong for hundreds of millions of years. We know that males today will sometimes select an inappropriate partner during the breeding season – a frog from a different species, a turtle, a fish or even an inanimate object. Now there is evidence that these mistaken attachments could be an ancient feature of frog reproduction, arising early in the amphibians’ evolution.

Frog mating is often hard to miss. In most species it involves a process called amplexus, in which males grip onto a female tightly for hours or days at a time until the eggs are fertilised. But there are plenty of records of male frogs grappling an unpromising target such as a frog from a different species or a dead individual. One explanation is that such mistakes are more likely to happen in species that breed in large numbers with a low ratio of females to males, and where multiple species occupy the same breeding pond.

Konrad Lorenz once observed a Shell Parakeet who grew amorous with a small celluloid ball. And many other animals exhibit mating behavior toward what researchers refer to as "biologically inappropriate objects." Bulls will treat almost any restrained animal as a receptive cow. Their general rule in life seems to be, "if it doesn't move away and can be mounted, mount it!"

During the early 1950s, researchers at Walter Reed Army Medical Center surgically damaged the amygdala (a region of the brain) in a number of male cats. These cats became "hypersexual," attempting to mate with a dog, a female rhesus monkey, and an old hen. Four of these hypersexual cats, placed together, promptly mounted one another.