PL/SQL Brain Teaser: when does a COMMIT not commit?

So your users make changes to tables (with great care and security, through your PL/SQL API), your app calls this procedure and that function. At some point along the way, a COMMIT statement is executed successfully in a user's session.

Yet that same user's session still has uncommitted changes!

Huh? How is this possible?

Think you know? Comment below!