This may be just a memory bug but for me it is anyway a subjective mystery.
About 15 years ago we were having at work a discussion that was most likely about some topic in computer science or formal logic. A colleague made a claim that I countered with a phrase which - to the best of my memory - belonged to Jules Verne’s character Pencroft from The Mysterious Island:
If it looks like a duck, walks like a duck and quacks like a duck then it is a duck.
(The real phrase, as I remembered it, ended with “… then I call it a duck” but I slightly edited it to better fit my argument).
My feeling at that time was that I had impromptu found a good quote from literature, so I think that I didn’t know about duck typing then. When I later encountered that term I felt pleased to have earlier made a similar analogy on my own. But there was no link to Jule Verne whereas I was certain that I hadn’t ever read American poet James Whitcomb Riley’s verse.
I searched for the phrase in The Mysterious Island with no success. I both used an online search as well as looked through two hardcopy volumes (two different Russian translations). I re-watched the Soviet film adaptation of that book. I considered that my memory may have failed me only partially and the phrase might be from another novel. I skimmed through the hardcopies of Five Weeks in a Balloon, In Search of the Castaways (Les Enfants du capitaine Grant) and Twenty Thousand Leagues Under the Sea that I had read as a teenager. All in vain.
Those attempts to resolve the mystery have been scattered over several years. The slight feeling of uneasiness caused by that puzzle is still with me.
I realize that the simple explanation may be that I had read an article about duck typing before the described discussion and the phrase had resonated with some of Pencroft’s utterances creating a strong and false association that remained while the weaker impression from the technical material faded.
Even in that case the fact that my memory could have let me down in such a way is a source of slight discomfort for me.
P.S. Here is the likely fragment that can be blamed for the false memory:
After leaving the region of bushes, the party, assisted by resting on each other’s shoulders, climbed for about a hundred feet up a steep acclivity and reached a level place, with very few trees, where the soil appeared volcanic. It was necessary to ascend by zigzags to make the slope more easy, for it was very steep, and the footing being exceedingly precarious required the greatest caution. Neb and Herbert took the lead, Pencroft the rear, the captain and the reporter between them. The animals which frequented these heights—and there were numerous traces of them—must necessarily belong to those races of sure foot and supple spine, chamois or goat. Several were seen, but this was not the name Pencroft gave them, for all of a sudden—“Sheep!” he shouted.
All stopped about fifty feet from half-a-dozen animals of a large size, with strong horns bent back and flattened towards the point, with a woolly fleece, hidden under long silky hair of a tawny color.
They were not ordinary sheep, but a species usually found in the mountainous regions of the temperate zone, to which Herbert gave the name of the musmon.
“Have they legs and chops?” asked the sailor.
“Yes,” replied Herbert.
“Well, then, they are sheep!” said Pencroft.