The Julian and Gregorian systems aren't quite the same I don't think, as the Julian system doesn't have the condition for century years, so all years divisible by 4 are leap years. Anyway, it's not unknown to have Feb 30, although the only one I can find was in Sweden in 1712. Possibly someone wrote it in good faith, either believing it was a so-called double leap year, or just not knowing about leap years at all.