I was working at the time on a laptop with a fully updated Windows 7, and around 200 GB of free hard-drive space. I was saving to my Dropbox folder. I had saved several times there without any problem. Then after the problem, I went to my desktop computer, unplugged it from the Internet before booting it up, and then retrieved the old project file (from the previous day) that was in the Dropbox folder on that machine. In fact, there were two files -- the original and one backup. But after copying those elsewhere, I plugged in the Internet, and when Dropbox synced up, the files both disappeared (as of course I knew they would). And yes -- I have since set up the automatic backup to an alternate location, which is a very good feature to use, I now realize!
It could be, as JohnnySix suggests, that some other program in the background interfered with the save process, although I haven't experienced this with any other save process on that machine.
Anyway, I still love Construct 2!
I got to thinking maybe frequent saving to the Dropbox folder was the problem. Not exactly sure how Dropbox does its thing, but I know it has to update the web copy after changes are made in the folder on the local drive. What if it happened to be doing that when another C2 save was attempted? Maybe that's not it, but the fact is, I was saving to the Dropbox folder when the problem occurred. Just to be safe, I decided not to do "working saves" to the Dropbox folder, but just to save there once at the end of a session. I used C2 (r83) a couple more hours yesterday, saving numerous times, and the problem in this report never repeated. I'm also doing auto-saves every 30 minutes to a different location than the project folder. Then the "end of session/ off-site" save to Dropbox.