Visa have just created this problem for themselves with their new subdomain system for visaoffers.eu. The subdomain is the three character code for the bank, so
[https aib.visaoffers.eu/]
is treated by Safari as the same as the subdomain:
[https boi.visaoffers.eu/]
These are two different banks and two different Visa cards. Safari only recognises them as one domain, so it can't automatically generate or save passwords multiple subdomains of this site. Obviously one doesn't want two different cards associated with one login/password.
regards,
Martin