When using OAuth, OpenID for login (together with native login on the site) there is the possibility of the same user having multiple accounts on a site. It is probably that the user who has Facebook, Twitter, Yahoo!, Google etc etc will forget which ID they used to login the site. I'm thinking of giving them the option of 'joining' their various accounts into one. This way their posts are in one place for them to manage.
Is there something flawed with this thinking?
The only flaw is assuming that one identity must equal one account.
As you can see here on stackoverflow, users can have two openid identities pointing to one account. Nothing stops you from allowing more identities per account on your site, and that's a good idea -- not because people will not have to remember what identity did they use, but because they will have a backup in case of one of the providers disappearing.
Of course, you could also create an option of joining existing accounts, but that's way more complicated, and I don't think it would be popular (because people rarely create two accounts just to have them associated).