I had that happen with AeroFS instead of Dropbox, simply by using a repo on one machine that was not synced to the contents of the other machine. This can happe

I had that happen with AeroFS instead of Dropbox, simply by using a repo on one machine that was not synced to the contents of the other machine. This can happen either if a machine doesn't finish sync to dropbox after using a repo in it, or the repo is used before the machine has finished syncing from dropbox. A simple "git commit" and close the laptop in a hurry as soon as the command finishes would likely do it.