When it's a donkey.
A study in 2004 analysed the remains of five horses found in the house of the Casti Amanti in Pompeii, all of which were well preserved by the volcanic ash produced in the erruption of AD 79. Analysis of the DNA showed four normal horses, and a highly unusual DNA in the fifth the researchers believed was of a breed of horse now extinct.
Susan Gurney, of the University of Cambridge, has revisited the original research. The fifth sample with the supposedly extinct DNA, contained, due to a laboratory error, the DNA of two animals: a donkey and a Herculaneum horse. In a slight alleviation of the orginal researchers' blushes, the donkey's DNA shows it is from the Somali strain, rather than the Nubian strain more common in Europe, showing that the Somali strain was present in Italy from at least Roman times.
Many thanks to Jonathan Badger for sending me this.