I've come across a strange behaviour when using Safari in "Private Browsing" mode (under Lion).
Recently, I'd visited a website which required logging in. Then I logged out, and after some time, tried to log in again. Somehow, my login name (which is an email address) was already in the respective field. "OK, that's strange, and it shouldn't happen," I thought and cleared Safari's Cache and Website Data. Interestingly, this wouldn't work either, and my login info was still accessible to Safari. I then went to Users/.../Library and trashed everything related to Safari that I could find. But to no avail. Now, at this moment I was surprised and a bit frustrated! Then I tried to access the same website using a second Mac. Login field remained unfilled in Safari there. Apparently, this issue is system-specific.
Obviously, login data had been stored somewhere as a cookie or in a plist file or whatever. Question is, why on Earth does this happen in PRIVATE BROWSING mode, and where exactly this data is being stored? Anyone?