A file can easily be corrupted if it is kept on Dropbox and you open it on a computer before the file is synced with all of the computers you use it on. Realmac will tell you not to keep project files on Dropbox without first compressing it, but if you follow these rules EXACTLY you should not have any issue:
The big thing to remember is when working on a laptop computer, that you be sure Dropobx has synced completely, I have actually had another corruption since I made that post and that was because I opened my laptop and just assumed it was synced, and it was not, killing the project file when I opened it.