It's a defect, it leads to inconsistent and unpredictable behavior. A better default behavior would have been to use a default radix (base 10) and have an option for auto-detected radix (e.g. strings that begin with 0x). Considering how rare octal formatted integer strings are these days (essentially non-existent) and how much more common 0 prefixed decimal formatted integer strings are, this is definitely not a useful feature.