One of the things Dropbox doesn’t do well is simultaneous editing. If you and a colleague are working on a document simultaneously, Dropbox will create a ‘Conflicted Copy’ which will appear in your Dropbox with the date and name of the author. It’s then up to you to bring the changes together into a single document. To be honest, you’re better off using a web tool such as Google Docs if you want to do simultaneous editing of files.
However, this also happens if you are working on a single document, say, on your laptop and desktop. The important thing is that you manually save and close your document at the end of each session. That way when you open it up on the new machine you know you’re dealing with the latest version of the file.