Steps to Replicate
Create a new Stack Overflow account, Test A, using OpenID.SE with the email
Create a new Programmers.SE account, Test B, using OpenID.SE with the email
Attempt to add the OpenID.SE login with the email
firstname.lastname@example.org the Stack Overflow account Test A.
The user would be prompted to merge the two Stack Exchange accounts or an error message would be displayed.
The user is redirected as though the login was added successfully, but it is not actually added.