Yes. To take a larger example action, creating a user could fail to be atomic if, say, it stored the username in a separate table from the user object, wrote th

Yes. To take a larger example action, creating a user could fail to be atomic if, say, it stored the username in a separate table from the user object, wrote the username first, then referenced it from the user object, but didn't roll back the username insert if the user object insert failed. Likewise, for the seemingly simpler example if establishing a friend relationship, you may be tracking that relationship in both directions, in which case one could fail and the other succeed.