Happen to me many times. It forced to start saving different file names with revisions:
Here, if I save the project in file format, close, open, save in folder again, close, open, it will show up, sometimes, a missing sprite inside the >Background folder, copying the sprite there, the file still opening normal.
The missing sprite always happen only for the >Background folder in this project.
My workaround is save in file every Release and save in folder sometimes, so, if something goes wrong, I can set backup and in the most hard case, use the source code to restore the project and folders setups.
The first sample file (the C2 samples) I tried to reproduce on Chrome,
<img src="http://dl.dropbox.com/u/47035927/temp/SCREENSHOT.png" border="0" />
I was using Chrome in privative navigation and it started a common navegation window, showing the error up.