Each time I log out and later back in to my Dropbox account especially from my mobile device, the system sends me a message that a new device has logged in. The notice needs me to confirm it was me by submitting a yes or no. The reason I think this may be a system design problem is that If I change a file on my main computer and then need to update it from my mobile device it won't merge the changes with the primary because since it cant tie the changes made to me and that the changes were made by another user. There are no new devices or other users.
As a solution to this problem I'd suggest tieing the devices loged in to the account by using the discrete and unique device IDs (im not aware of any device that wouldn't have one) as an check to verify the source of the change rather than relying on an email check to determine who the login was from. Using the device ID type of identification technique would in my view eliminate the "we can't merge the changes from another user" type of error which leaves the user with the option of either discarding the chages or saving a copy of the file. It ends up being a royal pain to manually update the primary file on the .com version of Dropbox.