You should read the article. It prominently features a very interesting description of precisely why their `canonicalise` function turned out to not be idempotent, even though it was meant to be.

That's exactly what they describe as the cause of the bug. They intended for the function to be idempotent but it wasn't because of a misunderstanding with the Python library spec.

Worse, it /did/ work that way in Python 2.4 but Python 2.5 stopped throwing an exception for invalid codepoints which broke the Twisted library which broke their canonicalization function.

I see you came here for the comments. You should check out the post. It seems relevant.