Symptoms

A user tries to log in using Google and after he/she clicks the 'Log in using Google' button the following error appears:

Cause

1. One of the possible causes may be because there is a user already created on the application which username is the full email address, instead of just the part before the @ sign.
2. Another cause is when the username and its email address (the part before the @ sign) doesn't match, i.e. 

In the example above, when the user logs in with Goggle using the email testing@example.com, JIRA will check if there's already a username 'testing'. If the username 'testing' doesn't exist, JIRA will automatically try to create it, however, this operation will fail because because there is another username which email is 'testing@example.com'.

Resolution

Edit this user's details and change its username removing the '@example.com' part.