Path into Integrity Constraint Violation

If a user has an account with a local Drupal login.
Then separately creates an SSO account on your IdP.
And usernames match.

The user_external_load() function will fail and they will find themselves hitting user_external_login_register() with an existing username with no authmap entry.
This will trigger a registration attempt for an existing user and error out the site with an integrity constraint, just for them, as nearly WSOD.

This violation did not exist in D6, and actually registered a new user on top of the old user with the same username.

The authmap creation discussed here #1280930: Create authmap entries for pre-existing Drupal users is highly related.