Scenario: You need to create a new user so you go to the AD controller and create or copy them. Then I went to my DirSync server and forced a "Start-OnlineCoexistenceSync" and then waited a few minutes for it to finish. Now at that point what I should have done was go to the on-premise Exchange server and created the new mailbox, and then ran DirSync. Instead since I already had the office365.com admin portal up I went ahead and assigned a license to the user since the object was already sync'd up on the cloud. When I went to submit a Remote Move Request of the mailbox from my on-premise server to the cloud, I got the friendly "The operation couldn't be performed because object
1. Create AD user
2. Create local mailbox
3. Force DirSync
4. Move mailbox to the cloud
5. Assign a license in the 365 admin portal
Steps 4 and 5 are interchangeable.
We create the mailbox locally first so that we retain the ability to move it back from the cloud to on-premise later if needed. DirSync does NOT replicate a GUID created initially from the 365 cloud back to your local AD.