In some documents, there are some mystakes about spelling Pupin's name (Mihailo-Mihajlo), but also with the date of his birth.

In some documents, there are some mystakes about spelling Pupin's name (Mihailo-Mihajlo), but also with the date of his birth. This problem came up because the name was written in old-slovenian language as Mihail, so there were two possibilities of spelling in the new serbo-croatian. Pupin himself, used to use both names at first, but later he used only Mihajlo. Proofs about this are in Pupin Museum in Idvor, in Serbian archive in Belgrade and some other places. Therefore, his will should be respected and only the name Mihajlo should be used in Serbian and Michael in English. He also added the name Idvorski, or the letter I, so we respect this desision too.

As the days of his birth can be find 27.9, 4.10, and lately 8 and 9.10. And as the year of his birth: 1854, 1857 and 1858. These problems came up because this particularity was signed in the birth-book by the Julian calendar which was used back then. It was signed by the number 237 that "on September 27, 1854 was born, and on October 4, was baptised Mihail, son of Konstantin Pupin and mother Olimpijada". Since there is 12 days difference between the old and new calendar, it means that Pupin was born 9.10.1954, according to the new calendar which is used almost everywhere in the world.