I'm not sure whether this is a known bug or not, but just in case:

I was wandering around the cities rankings (http://mirror.codeforces.com/ratings/cities), and when I tried to see the list of members from São Paulo by clicking on it, the system told me that city didn't exist.

I noticed it happens with and only with any city whose name contains diacritics: ´ ~ ^ etc
Examples:
http://mirror.codeforces.com/ratings/country/Poland/city/Gdańsk
http://mirror.codeforces.com/ratings/country/Brazil/city/São Paulo
http://mirror.codeforces.com/ratings/country/Brazil/city/Goiânia

(formatted like this for readability. You can also try http://mirror.codeforces.com/ratings/country/Poland/city/Gda%C5%84sk)

PS: The expected behavior is as follows: http://mirror.codeforces.com/ratings/country/Russia/city/Moscow

*Edit: Just realized that if you manually switch these characters, you can actually go to the right place! http://mirror.codeforces.com/ratings/country/Brazil/city/Sao Paulo ... maybe not too hard to fix :)