alt Hacker NewsWith web applications it's not particularly unusual, because the whole system stack can be quite heterogeneous. If one part of the system is doing authentication and the other part is actually doing the action then it can be a real problem when they interpret the input differently. Differences between proxy and web server interpretations of HTTP headers have been a source of multiple vulnerabilities, for example.
It's not particularly unusual in Python, because it's normal that important functionality is implemented in other languages by a diverse set of third parties.
That said, this isn't a security vulnerability, it's just a bug. To meet a reasonable threshold for being a security issue, you need to show a real system that has an issue caused by this, and then the vulnerability is in that system, rather than in Python.
I'll grudgingly allow that a buffer overflow or an SQL injection possibility - in a library advertised as safe against that kind of bug - is a security issue, because there's so much history of turning those into real exploits. But a choice of library or language that makes those bugs easier to write - the idna library, or C or PHP say, is not itself a security issue.