If you have a wallet for web passwords, but don't want to save there your mail passwords, everytime kmail is closed (or account configuration is changed) it will try to open the wallet, so kdewallet will ask you the password, and it will ask, for each password, if the password should be saved on kmail's config file. It is easy to hit enter and, since the default is not to save the password, you'll lose the password. And, if kde is closed with kmail open, it will lose all your passwords.
This is a really serious usability problem.

How to reproduce:
Activate kdewallet with some password, and close it.
Open kmail and create a pop account, tell it to save the password. After finishing, it will try to open kdewallet to save the password there. Cancel kdewallet so the pasword should be saved on kmail config file. A dialog will popup asking for confirmation, so click on "save". Close kmail.
Open kmail, close kmail, kdewallet password will be asked again. Cancel. The confirmation dialog will popup again. If you have created more than one account, one dialog for each account will popup.
Open kmail. Close kde. All passwords will be lost.