We have email address verification on our accounts. Users have to verify their email addresses when they create their accounts and when they update their email addresses on existing accounts. With new accounts, users cannot finish logging into the site until their email is confirmed (this will probably change eventually so that they may log in but be unable to do certain things). In the case of existing accounts, the new email is held in a "pending" state and the system continues to use the old email address until the new one is confirmed.
We also have a restriction that there cannot be more than one account with the same email address.
My question is what happens if someone creates a new account or updates an existing account with someone else's email (but can't confirm it) and the person who owns the email address tries to create an account.