The entries in the certificate drop-down (e.g. used for the signing certificate and encrypt-to-self certificate in Notepad or Sign/Encrypt dialog) are not sorted alphabetically which makes it difficult to find an entry in the list.
People with just a few own certificates, i.e. the majority of users, probably didn't notice or didn't care. But it's still a usability/accessibility problem and it's a major pain if one has a large number of test certificates.