Originally Posted by
zhanglin
Having different passwords on different sites is a must. It's better that you request a username change if possible when one site has security issues, i.g. compromised. That saves a lot of time and efforts for both sides: you and your other sites.
Same email address, same username might be a must when you register an account from an invite sent from a friend on another website. But that also causes troubles, say, you constantly get PMs about your failed login. Staff will also be a little upset when you have to many failed logins and they don't want to see their users being under attacks. Your email address might be under attack too.
On some services, when you have too many failed logins, your account will be locked and you can only unlock your account by password recovery.