That's because any number preceded by 0 is treated as an octal number, and 9 is an invalid octal number. [...] The silly thing is that hardly anyone uses octal nowadays, but it continues to be part of the C, C++, Java and PHP standards. The mistake is also very common.

There's not much way around it, he notes - the format's been in use for a long time now and is so ingrained in just about every C-based language out there that it's "too deeply imprinted in modern compiler DNA" to take out.