As a historical note, the Mercury period-vs-comma¹ bug probably wasn't a typo as such. In that era, programmers didn't type their programs; they wrote them on p

As a historical note, the Mercury period-vs-comma¹ bug probably wasn't a typo as such. In that era, programmers didn't type their programs; they wrote them on paper, using special printed forms² so that their intent would be clear to the professional typists who entered it. To prevent typos, everything would be typed at least twice; in the days before diff, there was special-purpose hardware for this.³ I can think of three possibilities for the programmer making such an error: actually writing it wrong (which seems unlikely to me because the mental context of writing a loop range doesn't lend itself to writing a single number); a skipping pen; or a locale issue — i.e. a programmer of European origin mixing up the characters. ¹ http://catless.ncl.ac.uk/Risks/9.54.html#subj1.1 http://catless.ncl.ac.uk/Risks/9.54.html#subj1.1 ² http://en.wikipedia.org/wiki/File:FortranCodingForm.png http://en.wikipedia.org/wiki/File:FortranCodingForm.png ³ http://en.wikipedia.org/wiki/Keypunch#IBM_056_Card_Verifier http://en.wikipedia.org/wiki/Keypunch#IBM_056_Card_Verifier