Third time I've encountered this problem over the years..
Every time I go to a new StackExchange site, I think that (because I'm a SO member) I should use the "login with StackExchange" option. It's listed first and I'm a member, right, so it must apply to me?
Every time I try "login with StackExchange" and enter my email & password -- I get told "No account with this email found".
Not very helpful or correct. I know I have an account with this email, I've just come from StackOverflow! There's nothing to suggest that if "login with StackExchange" is only applicable to some users.. or that my having possibly originally signed up via my Google account somehow makes me a second-class citizen.
I signed up 2+ years ago, I have scores & scores of accounts at different websites.. I shouldn't need to keep track of tedious implementation details of my account; & really just expect a friendly & easy signon. The experience here, however is below the generally very good level of usability, elsewhere throughout SO & StackExchange sites.
I'm after a user-level improvement in this area, not a technical-level explanation as to why doing this better is impossible.
At the very least it needs some explanatory text and/or links on the signup pages, to explain to perfectly genuine StackExchange account-holders why their apparently valid email & account are being rejected for signup purposes.